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A  TEXT-BOOK 

OF 

DENTAL  HISTOLOGY 


AND 


EMBRYOLOGY 


INCLUDING 


LABORATORY  DIRECTIONS 


BY 

FREDERICK  BOGUE  NOYES,  B.A.,  D.D.S. 

PROFESSOR    OF    HISTOLOGY,     NORTHWESTERN    UNIVERSITY    DENTAL   SCHOOL,     1896-1914 

PROFESSOR  OF  HISTOLOGY  AND  ORTHODONTIA,  COLLEGE  OF    DENTISTRY 

UNIVERSITY  OF  ILLINOIS,    1914 

THIRD  EDITION,  THOROUGHLY  REVISED 

WITH  A  CHAPTER  ON  THE  ABSORPTION  OF  THE  ROOTS  OF  TEETH 
By  NEWTON  GEORGE  THOMAS,  M.A.,  D.D.S. 

PROFESSOR  OF  HISTOLOGY,    NORTHWESTERN    UNIVERSITY    DENTAL  SCHOOL,    1917-1919 

SECRETARY  AND  PROFESSOP  OF  HISTOLOGY,  COLLEGE  OF  DENTISTRY,  UNIVERSITY 

OF  ILLINOIS,   1919 

WITH  343  ILLUSTRATIONS   AND  21    PLATES 


LEA  &  FEBIGER 

PHILADELPHIA   AND   NEW   YORK 


COPYRIGHT 

LEA  &  FEBIGER 

1921 


PRINTED   IN  U.  S.  A. 


Go  mg  Ifatbcr 

Dr. 


TUHbose  long  ano  active  professional  career  bas  been  DevoteD, 

witbout  personal  ambition  or  selfisb  advancement,  to 

tbe  QOO&  of  tbe  Dental  Profession,  and  wbose 

unselftsbness  anO  sacrifice  bave 

possible  all  tbat  1  bave  Done 

or  mas  accomplish. 


550717 
« — ^ 


PREFACE  TO  THE  THIRD  EDITION. 


THE  exhaustion  of  the  second  edition  of  this  work  has  afforded 
another  opportunity  for  careful  revision  of  the  text  and  illustra- 
tions, and  the  addition  of  some  important  material,  which  has 
been  developed  since  the  appearance  of  the  second  edition.  Many 
new  drawings  and  a  number  of  new  micrographs  have  been  pre- 
pared. The  chapters  on  the  lymphatics  of  the  dental  region  and 
the  absorption  of  the  roots  of  teeth  have  been  added,  and  the 
chapters  on  embryology,  greatly  enlarged. 

The  conditions  at  the  present  time,  and  especially  the  interest 
of  the  medical  profession  in  the  mouth  as  a  source  of  systemic 
infection  have  put  new  emphasis  on  the  teaching  of  histology,  and 
have  greatly  changed  the  attitude  of  the  dental  profession.  The 
need  for  a  thorough  knowledge  of  tissue  structure  and  function  is 
realized  as  it  never  has  been  before,  and  the  demand  for  thorough 
training  in  the  fundamental  biological  sciences  has  greatly  increased. 

The  present  interest  and  emphasis  of  the  profession  on  the  relation 
of  the  pulpless  tooth  to  systemic  diseases  has  somewhat  changed 
the  relative  distribution  of  the  text.  The  pages  devoted  to  the 
enamel  have  been  reduced,  those  devoted  to  the  dentin,  cementum 
and  supporting  tissues  increased,  and  the  chapter  on  the  lym- 
phatics added. 

The  work  is  primarily  intended  as  an  elementary  text-book  for 
dental  students,  rather  than  an  exhaustive  treatise  on  dental 
histology.  For  this  reason,  discussion  of  disputed  ideas,  presentation 
of  various  opinions,  and  reference  to  the  work  which  has  developed 
the  subject  have  been  largely  and  purposely  avoided.  It  is  the 
author's  opinion  that  it  is  better  for  the  student  to  get  a  clear  idea 
of  structure  that  he  can  use  as  a  basis  for  thinking,  rather  than  to 
be  left  with  a  hazy  impression  of  differences  of  opinion. 

In  the  preparation  of  this  (the  third)  edition  the  author  is 
specially  indebted  to  Dr.  Newton  G.  Thomas,  who  has  prepared 
and  written  the  chapter  on  the  Absorption  of  the  Roots  of  Teeth, 
and  to  Mrs.  N.  M.  Frain,  the  artist  for  the  department,  who  has 
made  the  illustrations.  F.  B.  N. 

CHICAGO,  1921. 

(V) 


CONTENTS. 


INTRODUCTION 17 

CHAPTER  I 

HOMOLOGIES 19 

CHAPTER  II 
THE  DENTAL  TISSUES 28 

CHAPTER  III 
THE  ENAMEL 37 

CHAPTER  IV 

THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 41 

CHAPTER  V 
CHARACTERISTICS  OP  THE  ENAMEL  TISSUE 63 

CHAPTER  VI 

THE  DIRECTION  OF  THE  ENAMEL  RODS  IN  THE  TOOTH  CROWN        .      .       77 

CHAPTER  VII 
THE  RELATION  OF  THE  STRUCTURE  TO  THE  CUTTING  OF  THE  ENAMEL        84 

CHAPTER  VIII 

THE  STRUCTURAL  REQUIREMENTS  FOR  STRONG  ENAMEL  WALLS        .      .       90 

CHAPTER  IX 
STRUCTURAL  DEFECTS  IN  THE  ENAMEL 113 

CHAPTER  X 

SPECIAL  AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 125 

CHAPTER  XI 
THE  DENTIN 135 

CHAPTER  XII 
THE  CEMENTUM 153 

(vi) 


CONTENTS  vii 


CHAPTER  XIII 
DENTAL  PULP 164 

CHAPTER  XIV 
THE  LYMPHATICS  OF  THE  DENTAL  REGION 181 

CHAPTER  XV 
INTERCELLULAR  SUBSTANCES 200 

CHAPTER  XVI 
BONE 209 

CHAPTER  XVII 
BONE  FORMATION  AND  GROWTH 216 

CHAPTER  XVIII 
PERIOSTEUM 222 

CHAPTER  XIX 
THE  ATTACHMENT  OF  THE  TEETH 230 

CHAPTER  XX 
THE  PERIDENTAL  MEMBRANE 237 

CHAPTER  XXI 
THE  CELLULAR  ELEMENTS  OF  THE  PERIDENTAL  MEMBRANE       .      .      .     250 

CHAPTER  XXII 
ABSORPTION  OF  TEETH 275 

CHAPTER  XXII I 
THE  MOUTH  CAVITY 288 

CHAPTER  XXIV 

BIOLOGICAL  CONSIDERATIONS  FUNDAMENTAL  TO  EMBRYOLOGY     .      .      .     298 

CHAPTER  XXV 
EARLY  STAGES  OF  EMBRYOLOGY 302 

CHAPTER  XXVI 

THE  DEVELOPMENT  OF  THE  TOOTH  GERM 321 

CHAPTER  XXVII 

THE  RELATION  OF  THE  TEETH  TO  THE  DEVELOPMENT  OF  THE  FACE    .     334 


viii  CONTENTS 

PART  II. 

DIRECTIONS   FOR  LABORATORY   WORK 

(TWENTY-FIVE  PERIODS  IN  THE  LABORATORY) 

PRELIMINARY 377 

PERIOD  I 382 

PERIOD  II 382 

PERIOD  III 384 

PERIOD  IV 387 

PERIOD  V 387 

PERIOD  VI 389 

PERIOD  VII 390 

PERIOD  VIII 391 

PERIOD  IX 391 

PERIOD  X 392 

PERIOD  XI 393 

PERIOD  XII 393 

PERIOD  XIII 394 

PERIOD  XIV 394 

PERIOD  XV 395 

PERIOD  XVI 395 

PERIOD  XVII 396 

PERIOD  XVIII 396 

PERIOD  XIX 397 

PERIOD  XX 398 

PERIOD  XXI 399 

PERIOD  XXII 400 

PERIOD  XXIII 401 

PERIOD  XXIV  401 


APPENDIX. 

CHAPTER  I 
THE    GRINDING    OF    MICROSCOPIC    SPECIMENS,    USING    THE    GRINDING 

MACHINE 403 

CHAPTER  II 
THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE 424 

CHAPTER  III 
GENERAL  HISTOLOGICAL  METHODS 430 

CHAPTER   IV 
FIXING  AGENTS  AND  STAINING  SOLUTIONS     ,  439 


INDEX  .  ....     447 


DENTAL  HISTOLOGY. 


INTRODUCTION. 

THE  development  in  knowledge  of  the  cell  has  had  a  most  pro- 
found effect  upon  the  entire  practice  of  medicine;  in  fact,  the 
progress  of  modern  medicine  has  dated  from  the  studies  of  cell 
biology,  the  germ  theory  of  disease  being  only  one  of  the  phases 
of  this  development.  In  terms  of  the  cell  theory  the  functions 
of  the  body  are  but  the  manifest  expression  of  the  activities  of 
thousands  or  millions  of  more  or  less  independent  but  correlated 
centers  of  activity.  If  these  centers  or  cells  perform  their  func- 
tions correctly,  the  functions  of  the  body  are  normal,  but  if  they 
fail  to  perform  their  office  or  work  abnormally,  the  functions  of 
the  body  are  perverted.  In  the  last  analysis,  then,  all  physiology 
is  cell  physiology,  all  pathology  cell  pathology.  To  modern  medi- 
cine, histology,  or  the  cell  structure  of  the  organs  and  tissues  of 
the  body,  together  with  cell  physiology,  is  the  rational  foundation 
of  all  practice.  This  is  as  true  for  the  dentist  as  for  the  physician 
in  regard  to  the  soft  tissues  of  the  mouth  and  teeth  that  he  is  called 
upon  to  handle.  With  caries  of  the  teeth,  the  disease  which  most 
demands  the  attention  of  the  dentist,  the  case  is  somewhat  different. 
Caries  of  the  teeth  is  an  active  destruction,  by  outside  agencies,  of 
a  formed  material  which  is  the  result  of  cell  activity,  the  teeth 
themselves  being  passive.  The  cellular  activities  of  organs  and 
tissues  of  the  body  may  have  an  influence,  but  this  is  only  in  pro- 
ducing those  conditions  of  environment  which  render  the  activities 
of  the  destructive  agent  efficient  in  their  action  upon  the  tooth 
tissues.  Though  the  dental  tissues  are  passive,  the  phenomena 
of  caries  can  only  be  understood  when  the  structure  of  the  tissues  is 
understood,  and  not  only  must  the  treatment  be  based  upon  knowl- 
edge of  the  structure  of  the  tissues,  but  the  mechanical  execution 
of  the  treatment  is  facilitated  by  that  knowledge  of  structure. 

In  the  preparation  of  cavities,  the  arrangement  of  the  enamel 
wall  is  determined  by  the  knowledge  of  the  direction  of  the  enamel 
2  (17) 


18  DENTAL  HISTOLOGY 

prisms  in  that  locality,  and  to  a  certain  extent  the  position  of 
cavity  margins  must  be  governed  by  the  knowledge  of  the  struct- 
ure of  the  enamel.  In  the  execution  of  the  work  a  minute  knowl- 
edge of  the  direction  of  enamel  rods  becomes  the  most  important 
element  in  rapidity  and  success  of  operation.  The  longer  the 
author  studies  and  teaches  the  structure  of  the  enamel  in  its  rela- 
tion to  the  structure  and  preparation  of  enamel  walls,  the  more 
he  finds  himself  using  this  knowledge  at  the  chair  in  daily  opera- 
tions. He  believes  that  nothing  will  do  more  to  increase  facility, 
rapidity,  and  success  of  operation  than  a  close  study  of  the  enamel 
structure. 

All  tissues  are  made  up  of  two  structural  elements — cells  and 
intercellular  substances.  The  cells  give  the  vital  characteristics, 
the  intercellular  substances  the  physical  character.  The  cells 
are  the  active  living  elements,  the  intercellular  substances  are 
formed  materials  produced  by  the  activity  of  the  cells,  and  more 
or  less  dependent  upon  them  to  maintain  their  quality,  but  they 
possess  no  vital  properties.  They  surround  and  support  the  cells, 
and  the  physical  characteristics  are  given  by  them.  An  under- 
standing of  the  relation  of  cells  and  intercellular  substances  in  the 
structure  and  function  of  tissues  is  absolutely  fundamental  to  the 
study  of  dental  histology,  and  should  be  acquired  in  a  thorough 
study  of  general  histology  before  the  subject  is  undertaken. 

At  the  time  the  first  edition  of  this  work  was  prepared  the  relation 
of  histologic  structure  of  the  enamel  to  the  mechanical  operation 
of  dentistry  was  receiving  special  attention  because  of  the  study  of 
cavity  form  for  the  prevention  of  the  recurrence  of  caries,  and  the 
changes  necessitated  in  cavity  preparation  because  of  this  study. 
This  phase  of  dental  histology  is  just  as  important  as  ever,  but  it  has 
been  so  generally  accepted  and  so  clearly  grasped  that  now  most  of 
the  applications  in  practice  are  taught,  where  they  properly  belong, 
in  Operative  Dentistry  and  the  Technique  of  Cavity  Preparation. 
In  this  edition,  therefore,  the  space  devoted  to  this  subject  is  greatly 
reduced. 

The  problem  of  the  pulpless  tooth  which  now  occupies  the  fore- 
most place  in  the  attention  of  the  dental  and  medical  professions 
emphasi/es  the  importance  of  the  histology  of  the  dentin  and 
cementum,  and  places  new  importance  on  the  relation  of  cellular  and 
intercellular  substances  in  the  tissue.  For  the  dental  student  this 
subject  should  be  given  more  careful  consideration  than  is  usual 
in  elementary  courses  of  general  histology. 


CHAPTER  I. 
HOMOLOGIES. 

Exoskeleton. — In  studying  the  organization  of  animal  forms  they 
are  found,  very  early  in  the  evolutionary  stages,  to  develop  some 
sort  of  a  framework,  or  skeleton,  to  support  and  protect  the  crea- 
ture. In  the  lower  and  earlier  forms  this  framework  is  formed 
entirely  of  some  sort  of  shell  upon  the  outside  of  the  creature,  and 
consequently  is  called  an  exoskeleton.  This  may  be  either  horny 
or  chitinous  in  nature,  as  in  the  insects,  crabs,  etc.,  or  it  may  be 
calcified,  as  in  the  shell-fish,  or  it  may  be  both.  The  exoskeleton 
serves  not  only  as  a  supporting  framework,  but  also  as  a  protection. 

Endoskeleton. — In  the  higher  forms  an  internal  framework,  or 
endoskeleton,  is  developed,  which  forms  the  scaffolding  to  support 
the  creature,  but  does  not  act  as  a  protection.  In  the  first  place, 
this  is  of  cartilage,  but  may  be  changed  into  bone. 

The  exoskeleton  is  a  product  of  the  skin  and  may  be  of  either 
epithelial  or  connective-tissue  origin,  or  from  both.  The  skin  is  made 
up  of  two  parts:  the  epithelial  covering  or  epidermis,  and  the  sup- 
porting connective-tissue  layer,  or  derma.  Both  layers  take  part 
in  the  formation  of  most  exoskeletal  structures.  In  the  hair,  the 
shaft  is  of  epithelium,  the  bulb  of  connective  tissue.  In  the  tooth, 
the  enamel  is  from  the  epithelium,  the  dentin,  from  connective  tissue. 
In  all  bony  structures  belonging  to  the  exoskeleton  the  bone  is 
formed  in  fibrous  tissue  and  is  never  preceded  by  cartilage.  Bony 
structures  belonging  to  the  endoskeleton  are  formed  from  cartilage. 
In  lower  forms  of  animals  they  remain  always  cartilage.  In  man  the 
cartilage  is  partly  converted  into  bone,  all  of  the  bones  of  the  endo- 
skeleton being  preceded  by  cartilage. 

The  first  trace  of  the  endoskeleton  is  found  in  the  lowest  form 
of  vertebrate,  the  Amphioxus  or  Lancit,  the  lowest  form  of  fish, 
and  appears  as  a  rod  or  notochord  in  the  dorsal  region.  There  is 
also  an  important  difference  in  the  nervous  organization  (Figs.  1 
and  2).  In  the  invertebrate  the  nervous  system  is  represented 
by  a  larger  or  smaller  ganglion  in  the  anterior  or  head  end,  corre- 
sponding to  the  brain;  this  is  dorsal  to  the  alimentary  canal.  From 
this  a  ring  passes  around  the  anterior  end  of  the  alimentary  canal 

(19) 


20 


HOMOLOGIES 


ENDOSKELETON 


21 


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22  HOMOLOGIES 

and  unites  with  a  chain  of  ganglia  ventral  to  it.  The  nervous 
system  of  the  invertebrate  then  is,  with  the  exception  of  the  brain 
ganglia,  ventral  to  the  alimentary  canal,  and  corresponds  to  the 
sympathetic  system  in  higher  animals.  It  will  be  noted  that  this 
arrangement  puts  the  nervous  system,  which  controls  the  activity 
of  the  individual,  in  the  most  protected  position.  The  invertebrate 
crawling  upon  the  ground  is  subject  to  attack  or  injury  from  above, 
but  it  may  be  cut  almost  in  two  before  the  nervous  system  is  reached. 

In  the  vertebrate  the  central  nervous  system  appears  as  a  chain 
of  ganglia  dorsal  to  the  alimentary  canal  and  notochord  (Fig.  2). 
This  difference  is  significant,  and  may  be  expressed  roughly  in 
this  way:  The  invertebrate  framework  is  an  outside  protecting 
shell,  upon  which  the  creature  depends  for  protection.  The  verte- 
brate framework  is  an  internal  structure  to  facilitate  motion  and 
give  support,  and  is  accompanied  by  a  development  of  the  nervous 
organization,  so  that  the  creature  protects  itself  by  more  rapid 
motion.  In  the  invertebrate  the  digestive  system  is  above  or 
dorsal  to  the  nervous  system,  in  the  vertebrate  the  nervous  system 
is  in  the  upper  position,  both  structurally  and  functionally. 

In  ascending  in  the  scale  of  organization  the  endoskeleton 
increases  in  importance  and  development,  while  the  exoskeleton 
decreases  in  importance  and  development. 

From  the  standpoint  of  comparative  anatomy  the  teeth  are 
not  a  part  of  the  osseous  system,  but  appendages  of  the  skin,  and 
are  to  be  compared  with  such  structures  in  the  body  as  the  hair 
and  the  nails.  The  teeth  are  a  part  of  the  exoskeleton,  and  their 
relation  to  the  bones  is  entirely  secondary  for  the  purpose  of 
strength,  the  bone  growing  up  around  the  tooth  to  support  it. 

Placoid  Scales.— In  the  skin  of  such  fishes  as  the  shark  and  the 
dog-fish  small  calcified  scales  are  found,  which  are  made  up  of  a 
conical  cap  of  calcified  tissue  like  enamel,  resting  on  a  cone  of 
dentin  which  contains  a  vascular  core  or  pulp.  These  are  sur- 
rounded by  a  basal  plate  of  tissue  like  cementum  into  which  the 
fibers  of  the  derma  are  imbedded.  Only  the  tips  of  these  scales 
project  through  the  skin.  These  are  the  structures  from  which 
the  teeth  have  been  derived  in  evolution. 

From  the  standpoint  of  development  the  mouth  cavity  is  to 
be  regarded  as  a  part  of  the  outside  surface  of  the  body  which  has 
been  enclosed  by  the  development  of  neighboring  parts,  and  the 
dermal  scales,  or  rudimentary  teeth,  which  are  found  in  the  skin 
covering  the  arches  forming  the  jaws,  have  undergone  special 


UOMOLOGY  AND  ANALOGY  23 

development  for  the  purpose  of  seizing  and  masticating  the  animal's 
food.  In  the  simplest  forms  there  is  only  a  development  in  size 
and  shape  of  these  scales,  and  they  are  supported  only  by  the  con- 
nective tissue  which  underlies  the  skin.  These  teeth  are  easily 
torn  off  in  the  attempt  to  hold  a  resisting  prey,  and  in  the  shark 
(Fig.  3)  they  are  continually  being  replaced  by  new  ones.  In  the 
more  highly  developed  forms  the  bone  forming  the  jaw  grows 
upward  around  the  bases  of  these  scale-like  teeth  to  support  them 
more  firmly  and  render  them  more  useful. 


FIG.  3. — Shark's  skul!  (Lamna  cornubica),  showing  succession  of  teeth. 

Homology  and  Analogy.— In  biology  structures  that  are  similar 
in  formation  and  origin  are  called  homologous.  Structures  that 
are  similar  in  function  are  called  analogous.  A  structure  or  organ 
may  be  both  homologous  and  analogous  to  another,  but  not  neces- 
sarily so.  For  instance,  the  wing  of  a  fly  is  analogous  to  the  wing 
of  a  bird,  because  they  are  used  for  the  same  purpose,  but  they  are 
not  homologous.  The  wing  of  a  bat  and  the  wing  of  a  bird  are 
both  analogous  and  homologous,  being  used  for  the  same  purpose, 
and  having  similar  structure  and  origin.  The  arm  of  man  is  homo- 
logous to  the  wing  of  a  bird,  but  not  analogous  to  it.  The  jaws  of 
a  crab  or  beetle  are  analogous  to  the  jaws  of  man,  but  they  are  not 
homologous  structures,  as  the  jaws  of  the  crabs  and  insects  are 
modified  legs.  The  teeth  are  said  to  be  homologous  to  the  dermal 


24 


HOMOLOGIES 


scales  of  certain  fishes,  and  to  the  appendages  of  the  skin,  such  as 
the  hair  and  nails,  because  they  are  similar  in  structure  and  origin 
(Plate  I). 

Comparison  of  Structure. — If  the  tooth  is  compared  with  the 
hair  in  this  way  this  will  be  better  understood.  The  hair  may  be 
considered  as  a  horny  structure  composed  of  epithelial  cells  resting 
upon  a  papilla  of  connective  tissue.  The  tooth  may  be  considered 
a  calcified  structure,  formed  by  epithelial  cells,  resting  upon  a  papilla 
of  connective  tissue,  which  is  also  partially  calcified. 


C°°°    •'i°<f'c.?'ol>'>°i0-o'r^'  "^    °~tf\ 


FIG.  4. — Development  of  the  hair:  Sc,  stratum  corneum;  SM,  stratum  malpighii, 
C,  derma;  Dr,  sebaceous  gland;  F,  follicles;  CZ,  central,  PZ,  peripheral  zone  of 
hair  germ;  HK,  hair  knob;  P,  beginning  the  formation  of  the  hair  papilla;  P', 
same  in  a  later  stage  of  development  when  it  has  become  vascular.  (Wiedersheim, 
Comparative  Anatomy  of  Vertebrates.) 

Comparison  of  Origin. — From  a  study  of  the  development  of  the 
tooth  and  the  hair,  the  similarity  of  their  origin  and  structure 
becomes  more  apparent. 

The  first  step  in  the  development  of  the  hair  is  a  thickening  of 
the  epithelium  at  a  point,  the  epithelial  cells  multiplying  and  grow- 
ing down  into  the  connective  tissue  below,  so  as  to  make  a  two- 


PLATE  I 


Comparison  of  Structure  of  Tooth  and  Hair. 


COMPARISON  OF  ORIGIN 


25 


layered  bag  or  cap,  the  connective  tissue  growing  up  in  the  form 
of  a  cone-shaped  papilla  into  the  cavity  of  the  cap  (Fig.  4).  The 
epithelial  cells  of  the  inner  layer,  next  to  the  connective  tissue, 
multiply  rapidly  and  develop  horny  material  and  are  pushed  out 
from  the  surface  of  the  skin  as  the  shaft  of  the  hair. 

In  the  development  of  the  tooth  there  is  at  first  a  thickening  of 
the  epithelium,  and  a  mass  of  epithelial  cells  like  that  forming  the 
hair,  but  larger,  grows  dowrn  into  the  connective  tissue  (Fig.  5). 
This  becomes  bulbous,  then  invaginated,  forming  a  two-layered 


FIG.  5. — Diagram  to  illustrate  development  of  a  tooth;  A,  inner  layer  of  enamel 
germ;  B,  outer  layer;  C,  remains  of  intermediate  cells;  D,  den  tin;  DL,  dental 
lamina;  E,  epithelium;  E.G,  enamel  germ;  En,  enamel;  F,  dental  furrow; 
L.D,  labiodental  furrow;  M,  connective-tissue  cells;  O,  odontoblasts;  P,  dentin 
papilla;  R. G,  reserve  germ;  V,  bloodvessel.  (Cunningham's  Anatomy.) 

cap.  The  two  layers  are  at  first  perfect  and  are  farther  from  the 
surface  than  the  epithelial  structure  which  develops  the  hair.  A 
cone-shaped  papilla  of  connective  tissue,  the  dental  papilla,  grows 
up  into  the  cavity  of  the  epithelial  organ  corresponding  to  the  bulb 
of  the  hair. 

The  inner  layer  of  epithelial  cells  produce  the  enamel,  the  outer 
layer  of  connective-tissue  cells,  covering  the  connective-tissue 
papilla,  develop  the  dentin,  leaving  the  pulp  inside  as  the  remains 
of  the  dental  papilla. 


26 


HOMOLOGIES 


Phylogeny  is  the  history  of  the  development  or  evolution  of  the 
species.  Ontogeny  is  the  development  of  the  individual.  In  homol- 
ogous structure  we  may  trace  the  similarity  in  their  origin,  both  in 


FIG.  6. — Changes  in  the  mandible  with  age;    buccal  and  lingual  view. 

ontogeny,  or  the  development  of  the  individual,  and  in  phylogeny, 
or  the  development  of  the  species. 

Relation  to  the  Bone. — The  relation  of  the  bones  of  the  jaws  to 
the  teeth  is  entirely  secondary  and  transient.     The  bone  grows 


RELATION   TO   THE  BONE  27 

up  around  the  roots  of  the  teeth  to  support  them,  and  is  destroyed 
and  removed  with  the  loss  of  the  teeth  or  the  cessation  of  their 
function.  In  this  way  the  development  of  the  alveolar  process 
appears  around  the  roots  of  the  temporary  teeth.  All  this  bone 
surrounding  their  roots  is  absorbed  and  removed  with  the  loss  of 
the  temporary  dentition,  and  a  new  alveolar  process  grows  up 
around  the  roots  of  the  permanent  teeth  as  they  are  formed.  This 
development  of  bone  around  the  roots  of  the  teeth  leads  to  the 
changes  in  the  shape  of  the  body  of  the  lower  jaw,  increasing  the 
thickness  from  the  mental  foramen  and  the  inferior  dental  canal 
upward  (Fig.  6)..  When  the  teeth  are  finally  lost  this  bone  is  again 
removed  and  the  body  of  the  jaw  is  reduced  in  thickness  from 
above  downward.  These  phenomena  have  an  important  bearing 
upon  the  causes  and  treatment  of  diseased  conditions  of  the  teeth, 
particularly  those  which  involve  the  supporting  tissues. 

From  the  dental  standpoint  it  is  important  to  note  that  the 
teeth  are  formed  first  and  the  bone  is  developed  to  support  them. 
The  use  of  the  teeth  through  occlusion  reacts  upon  the  formation 
of  bone.  The  study  of  anatomy,  as  well  as  direct  experiment,  has 
shown  that  muscular  function,  acting  through  occlusion,  affects 
the  development,  not  only  of  the  bone  of  the  alveolar  process,  jaws 
and  face,  but  of  the  entire  skull.  It  is  most  important  for  the 
student  to  realize  that  the  teeth  are  moving  with  reference  to  the 
skull  as  a  whole,  through  the  entire  period  of  development,  and,  in 
fact,  throughout  life. 


CHAPTER  II. 
THE  DENTAL  TISSUES. 

STUDY  of  the  structure  of  the  teeth  shows  that  all  teeth,1  from 
the  simplest  to  the  most  complex,  are  composed  of  but  four  tissues — 
enamel,  dentin,  cementum,  and  the  pulp,  or  formative  tissue  of 
the  dentin.  All  teeth  are  maintained  in  position  and  rendered  func- 
tionally useful  by  certain  supporting  tissues. 

Even  the  simplest  placoid  scales,  as  found  in  the  skin  of  the 
shark  and  dog-fish,  contain  these  four  tissues.  In  many  of  the 
specialized  forms  of  teeth  some  of  these  tissues  may  be  absent.  For 
instance,  in  the  bony  fishes  the  teeth  are  fastened  to  the  bone  by 
an  interlocking  of  bone  and  dentin,  forming  an  ankylosed  attach- 
ment, and  the  cementum  is  absent;  but  in  some  of  these  there  is 
also  a  slight  formation  of  cementum.  In  the  tusks  of  elephants 
during  the  functional  period  the  dentin  is  not  covered  by  enamel, 
but  when  the  tusk  first  erupted  there  was  a  slight  enamel  cap, 
which  was  at  once  broken  or  worn  off.  In  many  instances  the 
enamel  seems  to  be  entirely  absent,  and  for  that  reason  it  has 
sometimes  been  called  the  most  inconstant  of  the  dental  tissues, 
but  in  every  case  in  which  the  development  of  the  tooth  has  been 
studied  an  enamel  organ  has  been  found.  It  is  probably  much 
more  nearly  correct  to  consider  that  in  all  cases  enamel  is  formed, 
but  that  it  may  be  so  thin  and  transparent  as  to  be  very  difficult 
to  recognize,  and  very  soon  may  be  entirely  lost. 

FUNCTIONS   OF   THE   DENTAL   TISSUES. 

The  Enamel. — The  enamel  forms  a  hard  protecting  surface  or 
cap  especially  adapted  to  resist  abrasion.  It  is  the  hardest  animal 
tissue,  but  brittle  and  inelastic,  and  dependent  upon  the  support 
of  the  elastic  dentin  for  strength.  Its  function  is  to  resist  the 
abrasion  of  friction.  Its  arrangement  in  many  instances  is  found 
specially  modified  for  this  purpose. 

1  The  formation  of  a  satisfactory  definition  of  a  tooth  is  by  no  means  an   easy 
matter.     The  word  here  is  used  to  mean  teeth  that  are  derived  in  the  phylogenetic 
series  from  the  placoid  scale,  as  the  starting-point  of  evolution. 
(28) 


FUNCTIONS  OF  THE  DENTAL  TISSUES  29 

The  Dentin. — The  dentin  is  the  strong  elastic  tissue  forming 
the  great  mass  of  the  tooth,  and  gives  to  it  its  strength.  Teeth 
that  are  subjected  to  stress  and  force  are  often  made  up  of  dentin 
without  enamel.  If,  for  instance,  the  tusks  of  the  elephant,  used 
for  such  purposes  as  tearing  down  branches,  spading  up  the  ground, 
and  so  on,  were  made  up  entirely  of  enamel,  they  would  break  off 
the  first  time  they  were  locked  in  the  branches  or  driven  into  the 
ground,  but  the  elastic  dentin  gives  and  bends  and  will  stand  great 
stress.  The  teeth  of  many  animals  which  use  their  tusks  in  fighting 
are  constructed  on  the  same  plan.  Such  tusks  usually  have  an 
enamel  cap  when  they  first  erupt,  and  in  every  case  an  enamel 
organ  is  present  in  the  tooth  germ. 

The  Cementum. — The  cementum  furnishes  attachment  for  the 
connective-tissue  fibers  which  fasten  the  tooth  to  the  bone  or 
surrounding  tissues.  It  is  formed  on  the  enamel  and  dentin  both 
before  and  after  the  eruption  of  the  teeth  but  only  on  portions 
embedded  in  the  tissues  at  the  time  of  formation.  The  formation 
of  the  cementum  on  the  surface  of  the  root  fastens  the  surrounding 
connective-tissue  fibers  to  the  tooth.  The  fibers  are  calcified  along 
with  the  matrix  of  the  cementum  which  is  built  up  around  them. 
These  fibers  in  man  and  the  higher  animals  extend  to  the  bone  and 
the  surrounding  tissues  and  support  the  teeth  against  the  forces 
of  mastication  and  hold  the  surrounding  tissues  in  proper  relation 
to  the  teeth.  The  function  of  the  cementum  is  therefore  to  attach 
the  connective-tissue  fibers  to  the  surface  of  the  root. 

The  Pulp. — The  pulp  is  the  remains  of  the  formative  organ  of 
the  dentin.  In  teeth  of  continuous  growth  it  remains  actively 
functional  throughout  the  life  of  the  tooth,  but  in  teeth  of  limited 
growth,  after  the  typical  development  of  dentin,  it  becomes  func- 
tional again  only  in  response  to  irritations  which,  however,  may 
be  local  or  reflex.  The  pulp  performs  two  functions — a  vital  func- 
tion, the  formation  of  dentin,  and  a  sensory  function,  the  response 
to  thermal  change. 

Summary. — The  dental  tissues,  i.  e.,  enamel,  dentin,  cementum, 
and  pulp,  are  so  called  not  simply  because  they  are  found  in  the 
human  teeth,  but  because  all  teeth  are  composed  of  these  four 
tissues. 

It  is  true  that  in  comparative  dental  histology  considerable 
difference  exists  in  the  microscopic  structure  of  these  tissues  from 
the  teeth  of  different  animals,  but  certain  characteristics  are  very 
persistent  and  quite  characteristic  of  each. 


30  THE  DENTAL  TISSUES 

DISTRIBUTION   OF   THE   DENTAL   TISSUES. 

The  arrangement  and  distribution  of  the  dental  tissues  in  the 
structure  of  the  human  teeth  is  best  studied  in  ground  sections 
cut  longitudinally  through  the  entire  tooth  (Plate  II),  and  series 
of  transverse  sections  cut  through  the  roots.  For  this  purpose 
the  sections  should  not  be  too  thin  (from  10  to  20  microns).  For 
the  study  of  the  arrangement  of  the  cementum  and  dentin  in  the 
roots  at  least  three  transverse  sections  should  be  ground  from  each 
root,  one  from  the  gingival,  one  from  the  middle,  and  one  from  the 
apical  third. 

The  Enamel. — The  enamel  forms  a  cap  over  the  exposed  portion 
of  the  tooth.  Its  function  is  to  resist  the  abrasions  of  mastication. 
It  gives  the  detail  of  crown  form  to  the  tooth.  It  extends  to  the 
gingival  line,  and,  except  in  old  age,  is  covered  in  the  gingival 
portions  by  the  epithelium  of  the  gingivae  which  lies  in  contact 
with  it  but  is  not  attached  to  it.  It  is  thin  in  the  gingival  portion 
and  is  normally  overlapped  slightly  by  the  cementum  at  the  gingival 
line.  It  extends  farther  apically  on  the  labial  and  lingual,  and  buccal 
and  lingual,  than  upon  the  proximal  surfaces,  especially  on  the 
incisors,  cuspids,  and  bicuspids.  It  is  thickest  in  ,the  occlusal 
third  of  the  axial  surfaces,  and  on  the  occlusal  surfaces  of  the 
molars  and  bicuspids,  especially  over  the  cusps.  In  the  incisors 
and  cuspids  it  is  thickest  in  the  occlusal  third  on  the  labial  and  over 
the  marginal  ridges  on  the  lingual.  The  dento-enamel  junction, 
though  not  parallel  with  the  surface  of  the  enamel  is  usually  curved 
in  the  same  direction  except  near  the  cusps  in  molars  and  bicuspids, 
where  the  curve  is  sometimes  reversed,  apparently  to  give  greater 
thickness  of  enamel  where  resistance  to  wear  is  most  needed. 

In  the  molars  and  bicuspids  the  dento-enamel  junction  in  the 
occlusal  thirds  on  the  buccal  and  lingual  is  usually  curved  in  the 
opposite  direction.  That  is,  while  the  surface  of  the  enamel  is 
convex,  the  surface  of  the  dentin  is  concave.  It  will  be  seen  that 
this  not  only  gives  a  greater  thickness  to  the  enamel  in  the  region 
which  will  resist  abrasion,  but  also  gives  it  a  firmer  seat  upon  the 
dentin.  (Study  illustrations  in  Chapter  IX.)  The  dento-enamel 
junction  is  seldom  a  smooth,  even  surface,  but  will  appear  scalloped 
in  sections,  projections  of  dentin  extending  between  projections 
of  enamel  (Fig.  7).  In  three  dimensions  this  means  that  rounded 
projections  of  the  enamel  rest  in  rounded  depressions  of  the  dentin 
surface,  and  pointed  projections  of  the  dentin  extend  between  the 


PLATE  II 


Cm. 


Ground  Section  of  a  Canine. 

E,  enamel;  Cm,  eementum;  D,  dentin;  PC,  pulp  chamber;  De,  dento- 
enamel  junction;  Eil,  enamel  defect;  6',  junction  of  enamel  and  eementum 
at  the  gingival  line;  Gt,  granular  layer  of  Tomes.  (Reduced  from  a  photo- 
micrograph made  in  three  sections.) 


DISTRIBUTION  OF  THE  DENTAL  TISSUES 


31 


rounded  projections  of  the  enamel.  This  is  similar  but  much  less 
marked  than  the  interlocking  of  the  papilla  of  connective  tissue 
with  the  projections  of  the  Malpighian  layer  of  stratified  squamous 
epithelium  of  the  skin  and  mucous  membrane.  In  some  cases  these 


FIG.  7. — Dento-enamel  junction. 

projections  of  dentin  into  the  enamel  may  be  quite  marked.  This 
scalloping  of  the  dento-enamel  junction  gives  a  stronger  attach- 
ment of  the  enamel  to  the  dentin,  and  accounts,  partially  at  least, 
for  the  difference  that  is  observed  in  the  ease  with  which  enamel 


32  THE  DENTAL   TISSUES 

can  be  removed  from  the  dentin  in  the  preparation  of  roots  for 
crowns.  Where  the  two  tissues  join  with  smooth  surfaces  the 
enamel  can  be  comparatively  easily  cleaved  away;  where  the 
scalloping  is  marked  it  is  removed  with  much  greater  difficulty. 

The  Dentin. — The  dentin  gives  the  strength  to  the  tooth.  This 
should  never  be  lost  sight  of  in  operations,  and  sound  dentin  should 
always  be  conserved  to  the  greatest  possible  extent  in  the  prepara- 
tion of  cavities.  That  the  function  of  the  dentin  is  to  give  strength 
will  be  seen  more  clearly  from  a  comparative  study  of  teeth  modified 
for  special  functions.  The  dentin  forms  the  greatest  mass  of  the 
tooth,  the  type  form  being  determined  by  it.  The  cusps  and  ridges, 
although  different  in  form,  are  still  represented  in  the  dentin  as 
well  as  the  number  and  shape  of  the  roots,  while  the  detail  of  the 
form  of  the  roots  is  modified  by  the  addition  of  the  cementum  on 
the  surface. 

The  dentin  forms  a  layer  of  comparatively  even  thickness  sur- 
rounding the  central  cavity  or  pulp  chamber,  which  is  occupied 
by  the  formative  organ.  From  this  cavity  a  great  number  of-' 
small  tubules  extend  through  the  calcified  dentin  matrix  to  the 
surface  under  the  enamel  and  cementum.  In  the  crown  portion 
the  course  of  these  tubules  is  characteristically  curved  like  the 
letter  S  or  /,  so  that  the  tubules  tend  to  enter  the  pulp  chamber 
at  right  angles  to  the  surface  and  to  end  under  the  enamel  at  right 
angles  to  the  dento-enamel  junction  (Plate  II).  On  closer  study 
these  tubule  directions  will  be  found  to  be  more  complicated,  but 
in  studying  the  distribution  of  dentin  they  should  be  noted.  In 
the  root  portion  the  tubules  are  usually  comparatively  straight, 
that  is,  without  the  double  curve,  and  are  at  about  right  angles 
to  the  axis  of  the  canal. 

The  outer  layer  of  dentin  under  low  magnification  presents  a 
peculiar  granular  appearance,  which  is  specially  apparent  under 
the  cementum.  This  is  known  as  the  granular  layer  of  Tomes, 
and  is  caused  by  irregular  spaces  in  the  dentin  matrix  which  com- 
municate with  the  dentinal  tubules. 

The  Cementum. — The  cementum  covers  the  dentin  in  the  root 
portion,  and  in  most  cases  slightly  overlaps  the  enamel  at  the 
gingival  line.  This  is  not  always  true,  for  in  some  cases  it  just 
meets  the  enamel,  and  in  others  there  is  a  space  where  the  dentin 
is  uncovered  between  the  enamel  and  the  cementum  (Fig.  8).  It 
has  not  been  positively  determined  whether  this  can  ever  be  con- 
sidered a  normal  condition,  and  the  author  has  some  reason  to 


DISTRIBUTION  OF  THE  DENTAL   TISSUES 


33 


suppose  that  the  sections  showing  this  condition  were  from  teeth 
from  which  the  gums  had  receded  and  the  cementum  was  destroyed. 
The  sensitiveness  which  is  so  marked  in  some  cases,  where  the 
gums  have  receded  beyond  the  gingival  line,  is  probably  due  to 
the  loss  of  cementum  and  the  uncovering  of  the  granular  layer  of 
Tomes. 

The  cementum  is  thin  and  structureless  in  appearance  in  the 
gingival  portion  when  viewed  with  low  powers,  but  becomes  thicker 


FIG.  8. — Gingival  line,  showing  the  relation  of  enamel  and  cementum. 

in  the  apical  third.  In  the  thicker  portions  irregular  spaces  (lacuna?) 
with  radiating  canals  (canaliculi)  are  seen.  In  life  these  spaces 
contain  living  cells  (the  cement  corpuscles),  which  correspond 
to  the  bone  corpuscles  found  in  the  lacunae  of  bone.  Upon  the 
convex  surfaces  of  the  root  the  cementum  is  thin;  upon  the  con- 
cave surfaces  it  is  thicker.  This  increases  with  age,  and  so  the 
continuous  formation  of  cementum  tends  to  round  the  outlines 
of  the  roots  and  to  unite  them  where  they  approach  each  other. 
The  fibers  which  are  built  in  the  cementum  are  often  imperfectly 
3 


34  THE  DENTAL  TISSUES 

calcified,  especially  where  the  layers  are  thick,  so  that  in  the  ground 
sections  they  may  often  be  easily  mistaken  for  canals,  because  the 
imperfectly  calcified  fiber  has  shrunk  in  the  preparation. 

ADAPTATION  IN  THE  DISTRIBUTION  OF  DENTAL  TISSUES. 

If  the  teeth  of  mammals  are  studied  in  a  comparative  way 
many  modifications  will  be  found  in  the  relative  amount  and  dis- 
tribution of  the  dental  tissues,  adapting  the  tooth  to  perform  special 
functions.  A  study  of  these  modified  or  specialized  teeth  will 
give  a  better  understanding  of  the  functions  of  the  tissues  in  the 
tooth.  The  human  tooth  may  be  taken  as  a  type  of  omnivorous 
tooth,  and  the  arrangement  and  distribution  of  its  tissues  has 
already  been  described. 

Teeth  of  Continuous  Growth. — In  many  animals  the  teeth  or 
some  special  teeth  are  developed  as  weapons,  or  as  implements 
to  aid  in  securing  food.  It  is  usually  the  cuspid  teeth  that  show 
this  modification,  as  in  the  tusks  of  the  boar  and  many  species 
of  the  carnivora,  the  tusks  of  the  walrus,  and  other  examples. 
In  the  case  of  the  elephant  the  incisors  have  been  developed  in  the 
same  way.  Whenever  the  teeth  have  been  developed  in  size  for 
uses  which  require  strength  and  the  ability  to  withstand  stress 
and  strain,  the  increase  in  size  is  by  development  of  the  mass  of 
dentin.,  the  enamel  often  being  entirely  lost  during  the  functional 
period.  If  these  teeth  were  composed  chiefly  of  enamel  they  would 
be  too  brittle.  These  tusks,  which,  as  in  the  case  of  the  elephant, 
sometimes  reach  a  weight  of  many  hundreds  of  pounds,  are  usually 
deeply  embedded  in  the  bone,  and  the  concealed  portion  is  covered 
with  a  layer  of  cementum  which  attaches  the  fibers,  holding  them 
to  the  bone,  but  they  retain  a  conical  pulp  in  a  cone-shaped  pulp 
chamber  at  the  base  of  the  tooth,  which  continues  to  form  dentin. 
The  tooth  is  pushed  out  of  the  socket,  as  the  shaft  of  the  hair  is 
pushed  out,  by  the  multiplication  of  cells  covering  the  bulb.  In 
this  way  the  size  of  the  tooth  is  maintained  as  the  exposed  and 
functional  part  is  worn  off.  Strength  and  elasticity  are  required, 
therefore  the  dentin  is  developed.  The  cementum  which  is  formed 
on  the  embedded  portion  for  attachment  of  fibers  is  worn  off  as 
soon  as  it  is  exposed  to  friction. 

Chisel  Teeth. — The  incisors  of  the  rodents,  as  rats,  mice,  squirrels, 
and  beavers,  present  an  interesting  modification  for  a  special 
function.  These  teeth  are  used  as  chisels  for  cutting  hard  sub- 


ADAPTATION  IN  DISTRIBUTION  OF  DENTAL   TISSUES     35 

stances,  as  wood,  shells  of  nuts,  etc.  Here  strength  and  hardness 
are  required.  The  dentin  is  increased  by  the  continual  function 
of  a  conical  persistent  pulp  which  continues  to  form  dentin,  and 
the  enamel  organ  is  carried  down  into  the  socket,  to  the  base  of 
the  dental  papilla,  on  the  labial,  instead  of  stopping  at  the  gingival 
line,  as  in  the  human  incisors.  In  this  position  it  continues  to  build 
enamel  on  the  labial  side  of  the  dentin.  The  enamel  rods,  instead 
of  being  straight,  are  twisted  about  each  other  in  a  complicated 
fashion,  giving  the  maximum  of  hardness.  As  the  incisors  work 
against  each  other  by  the  movements  of  the  jaw,  the  dentin  is 
worn  off  on  the  lingual  side  and  the  enamel  kept  in  the  form  of  a 
chisel  edge.  There  is  also  a  modification  of  the  temporomandibular 
articulation,  allowing  the  lower  jaw  to  move  forward  and  back  as 
well  as  up  and  down,  but  not  laterally,  so  that  the  lower  incisors 
can  be  closed  either  lingually  or  labially  to  the  upper,  and  in  this 
way  both  the  upper  and  the  lower  incisors  are  made  to  sharpen 
each  other  in  use.  In  this  case  there  is  need  for  both  strength 
and  hardness,  and  both  dentin  and  enamel  are  continuously  being 
formed  at  the  base  of  the  tooth  embedded  in  the  socket,  and  the 
cementum  is  formed  over  the  embedded  portions  as  the  medium 
of  attachment. 

Grinding  Teeth. — In  a  grinding  tooth,  as  in  the  molar  of  the 
horse  and  cow,  and  in  a  much  more  complicated  form  in  the  elephant, 
the  three  tissues — enamel,  cementum,  and  dentin — are  arranged 
so  as  to  form,  by  the  different  rapidity  of  abrasion,  corrugated 
grinding  surfaces  like  millstones.  The  conditions  can  be  under- 
stood if  it  is  remembered  that  the  cusps  in  the  dentin  are  very 
high,  and  are  covered  by  a  comparatively  thin  layer  of  enamel. 
After  the  enamel  is  formed,  and  while  the  tooth  is  embedded  in  its 
crypt  in  the  bone,  cementum  is  formed,  covering  the  surface  and 
filling  up  the  hollows  between  the  cusps,  so  that  the  crown  when 
it  first  erupts  is  rounded,  with  enamel  showing  only  at  the  tips  of 
the  cusps.  As  soon  as  the  tooth  wears,  the  tip  of  the  enamel  is 
worn  through,  so  that  the  circumference  of  the  crown  shows  first 
cementum,  then  enamel,  then  dentin,  then  enamel,  then  cementum, 
then  enamel,  and  so  on.  The  foldings  of  the  enamel  often  become 
very  complicated,  but  the  most  complicated  forms  can  be  under- 
stood in  this  way. 

Descriptive  Terms. — In  describing  the  structure  of  the  teeth  and 
the  arrangement  of  the  structural  elements  of  the  tissues,  direc- 
tions are  described  with  reference  to  three  planes:  The  mesio- 


36  THE  DENTAL  TISSUES 

disto-axial  plane  passing  through  the  center  of  the  crown  from 
mesial  to  distal  and  parallel  with  the  long  axis  of  the  tooth. 

The  bucco-linguo-axial  plane,  a  plane  passing  through  the 
center  of  the  crown  from  buccal  to  lingual  and  parallel  with  the 
long  axis  of  the  tooth. 

The  horizontal  plane  at  right  angles  to  the  axial  planes. 


CHAPTER  III. 
THE  ENAMEL. 

ENAMEL  may  be  defined  as  the  hard,  glistening  tissue  covering 
the  crowns  of  the  teeth  in  man  and  most  mammals.  It  is  the 
hardest  animal  substance  and  contains  less  organic  matter  than  any 
other  tissue  of  the  body. 

Histogenesis. — The  enamel  is  formed  by  the  epithelial  cells  of  the 
inner  tunic  of  the  enamel  organ.  After  the  tissue  is  formed  the  cells 
which  produced  it  are  destroyed  and  the  tissue  is  left  as  a  formed 
material  covering  the  dentin. 

Structural  Elements. — -The  enamel  is  composed  of  two  structural 
elements:  (1)  The  enamel  rods,  or  prisms.  (2)  A  calcified  sub- 
stance which  unites  the  rods  into  a  continuous  structure  called  the 
cementing,  or  interprismatic  substance. 

The  enamel  differs  from  all  other  calcified  tissues: 

1.  In  origin. 

2.  In  degree  of  calcification. 

3.  In  relation  to  its  formative  organ. 

4.  In  the  form  of  the  structural  elements  of  the  tissue. 

It  is  well  to  emphasize  these  points  of  difference,  for  throughout 
dental  and  medical  writing,  reasoning  by  analogy  from  bone  con- 
ditions to  tooth  conditions,  and  especially  to  changes  in  the  enamel, 
is  often  found.  For  instance,  the  argument  has  been  made  that 
because  there  may  be  changes  in  the  bones  in  pregnancy,  "  softening" 
of  the  teeth  would  be  expected.  Many  similar,  though  less  crude, 
arguments  would  not  be  made  if  it  were  remembered  that  histo- 
logically,  histogenetically,  physiologically,  and  morphologically  the 
enamel  stands  alone. 

Origin. — The  enamel  is  the  only  calcified  tissue  derived  from  the 
epithelium.  All  other  calcified  tissues  are  connective  tissues. 
Histogenetically,  then,  the  enamel  is  ultimately  derived  from  the 
epiblastic  germ  layer,  while  all  other  calcified  tissues  arise  from  the 
mesoblast.  Thus,  even  at  the  first  step  in  the  differentiation  of  the 
cells,  enamel  is  different  and  independent  from  bone,  cementum, 
or  dentin.  It  is  natural,  therefore,  to  find  the  enamel  differing 
from  bone  in  every  other  respect.  On  the  other  hand,  the  relation 

(37) 


38  THE  ENAMEL 

of  the  enamel  to  the  epithelium  becomes  more  and  more  apparent. 
For  instance,  imperfections  in  the  structure  of  the  enamel  during 
its  formation  are  most  likely  to  be  produced  by  systemic  conditions 
which  affect  the  epithelium.  The  eruptive  fevers  occurring  during 
enamel  formation  often  produce  imperfections  of  structure.  Scarlet 
fever  is  most  pronounced  in  its  epithelial  effect,  causing  loss  of  skin, 
loss  of  living  epithelium  of  the  alimentary  tract,  and  often  loss  of 
hair,  and  is  likewise  most  likely  to  produce  pitted  teeth  or  hypoplasia 
of  the  enamel.  In  other  words,  the  same  poison  which  is  produced 
by  the  germ  of  scarlet  fever  causes  the  death  of  epithelial  cells, 
of  the  skin,  of  the  hair  bulb,  of  the  mucous  membrane,  and  of  the 
enamel  organ. 

The  most  recent  work  of  Dr.  Black  shows  the  brown  and  mottled 
enamel  of  certain  localities  to  be  found  associated  with  greatly 
freckled  skin.  Enamel  therefore  must  be  considered  as  epithelial 
in  origin  and  ultimately  from  the  epiblast,  while  all  other  calcified 
tissues  are  connective  tissue  and  ultimately  of  mesoblastic  origin. 

Degree  of  Calcification. — The  enamel  is  by  far  the  hardest  animal 
tissue.  Chemically  it  is  composed  of  water,  calcium  phosphate, 
carbonate,  and  a  small  amount  of  fluoride,  magnesium  phosphate, 
and  a  trace  of  other  salts.  Normally  it  should  contain  no  organic 
matter.  Von  Bibra  gives  the  following  analysis: 

Calcium  phosphate  and  fluoride 89.82 

Calcium  carbonate 4.37 

Magnesium  phosphate 1.34 

Other  salts 0.88 

Cartilage 3.39 

Fat 0.20 

It  is  very  difficult  to  obtain  enamel  for  chemical  analysis  entirely 
free  from  dentin,  and  small  portions  of  dentin  clinging  to  it  are 
probably  responsible  for  some  of  the  organic  matter  given  in  the 
above  analysis. 

In  all  the  older  analyses  the  enamel  was  said  to  contain  95  to 
97  per  cent,  of  inorganic  matter  and  3  to  5  per  cent,  of  organic 
matter,  while  the  percentage  in  dentin  was  given  as  72  per  cent, 
of  inorganic  and  28  per  cent,  of  organic,  and  in  bone  as  68  per  cent, 
inorganic  and  32  per  cent,  organic  (dry  compact  bone).  This  in 
itself  shows  an  enormous  difference  in  the  degree  of  calcification 
between  enamel  and  the  other  hard  tissues,  but  the  results  of  more 
recent  work  are  still  more  remarkable.  In  most  of  the  original 
studies  of  the  chemical  composition,  the  enamel  was  broken  into 


DEGREE  OF  CALCIFICATION  39 

small  pieces  and  dried  for  some  time  at  a  temperature  above  the 
boiling-point  of  water,  to  drive  off  all  the  moisture.  The  dry 
enamel  was  weighed  and  then  ignited,  and  the  loss  in  weight  taken 
as  the  amount  of  organic  matter.  In  1896  Mr.  Charles  Tomes,1 
of  London,  published  the  results  of  his  chemical  analysis  of  enamel 
in  which  he  showed  that  a  large  part  of  the  loss  of  weight  in  ignition 
was  due  to  the  loss  of  water.  He  carried  out  ignition  in  tubes  to 
collect  the  products  of  combustion,  and  found  that  between  red 
and  white  heat  from  2  to  3  per  cent,  of  water  was  given  off.  This 
occurred  suddenly  and  with  almost  explosive  violence,  blowing 
large  pieces  to  fragments.  While  this  did  not  account  entirely 
for  all  of  the  matter  previously  considered  organic,  the  character 
of  the  product  of  combustion  and  the  observation  of  the  material 
during  ignition  led  him  to  conclude  that  the  remaining  portion 
was  due  to  the  dentin  adhering  to  the  enamel,  and  that  the  enamel 
contained  not  more  than  a  trace  of  organic  matter. 

Dr.  Leon  ^^Tilliams  attacked  the  problem  from  the  microscopic 
and  microchemical  side,  and  was  forced  to  the  conclusion  that 
normal  enamel  contains  no  organic  matter.  No  trace  of  organic 
matter  can  be  found  in  sections  of  enamel  by  staining.  And  if 
the  enamel  is  dissolved  by  acid  and  the  progress  observed,  not  a 
trace  of  organic  matrix  can  be  found.  The  conclusion  is  therefore 
imperative  that  enamel  is  composed  entirely  of  inorganic  matter, 
which  has  been  deposited  and  calcined  in  the  form  of  the  tissue  by 
the  formative  cells.  In  other  words,  enamel  is  formed  material 
produced  by  cells  and  laid  down  in  a  definite  structure,  but  it  con- 
tains no  organic  matrix,  while  all  other  calcified  tissues  are  composed 
of  an  organic  matrix  of  ultimate  fibrous  and  gelatin-yielding  char- 
acter, in  which  inorganic  salts  are  deposited  in  a  weak  chemical 
combination,  and  living  cells  are  retained  in  spaces  of  the  formed 
material. 

If  bone  or  dentin  is  subjected  to  the  action  of  acid,  the  com- 
bination between  the  organic  and  inorganic  matter  is  broken  up 
and  the  inorganic  matter  dissolved,  leaving  the  organic  portion, 
which  yields  gelatin  when  boiled  in  water,  in  the  form  of  the  original 
tissue.  If  enamel  is  treated  with  acid  the  cementing  substance 
between  the  rods  is  first  attacked  and  is  dissolved  more  rapidly, 
then  the  rods  are  attacked  from  their  sides,  and  finally  the  tissue 
is  entirely  destroyed,  leaving  no  trace  of  structure.  Apparently 
the  greater  the  dilution  of  the  acid  the  greater  will  be  the  extent 

1  Journal  of  Physiology. 


40  THE  ENAMEL 

of  the  solution  of  the  cementing  substance  before  the  rods  are 
destroyed. 

If  bone  or  dentin  is  burned  or  ignited,  the  organic  matter  will 
be  driven  off  and  the  inorganic  portion  will  be  left  in  the  form  of 
the  tissue,  still  showing  its  structure.  If  enamel  is  ignited,  water 
of  combination  and  whatever  foreign  matter  has  clung  to  the  pieces 
is  given  off,  but  the  form  of  the  tissue  is  unchanged.  To  illustrate 
the  difference  by  a  crude  comparison :  Bone  matrix  may  be  likened 
to  a  piece  of  cloth  into  which  inorganic  salts  have  been  deposited 
until  it  has  become  stiff  and  rigid,  but  the  web  of  the  cloth  is  still 
seen.  The  salts  may  be  dissolved  out  and  the  cloth  left,  or  the  cloth 
may  be  burned  out  and  the  salts  left.  The  enamel  may  be  compared 
to  a  fossil  in  which,  by  molecular  change,  the  organic  matter  has 
been  removed  and  inorganic  matter  substituted,  so  that  no  organic 
matter  remains,  but  the  structure  is  preserved.  If  the  inorganic 
salts  were  dissolved,  no  trace  of  structure  would  remain.  On  the 
other  hand,  by  ignition,  nothing  but  wrater  can  be  driven  off. 

Relation  to  the  Formative  Tissue. — The  enamel  is  produced  by 
epithelial  cells,  which  are  lost  and  destroyed  after  the  tissue  is 
completed.  Any  such  thing,  therefore,  as  a  vital  change  in  the 
tissue  is  biologically  unthinkable.  After  the  enamel  is  formed  it 
can  be  changed  only  by  chemical  and  physical  action  of  its  envi- 
ronment. 

All  other  calcified  tissues  are  formed  by  connective  tissue,  and 
remain  in  vital  relation  with  connective  tissue  of  undifferentiated 
character.  Bone  and  dentin  matrix  are  therefore  simply  calcified 
intercellular  substances  containing  living  cells  in  the  spaces  of  the 
matrix  which  maintain  its  chemical  quality.  A  change  in  the 
character  or  amount  of  the  matrix  might  possibly,  therefore,  be 
brought  about  by  the  vital  activity  of  these  cells.  Moreover, 
the  formed  matrix  is  always  in  vital  relation  with  undifferentiated 
connective  tissue,  which  may  at  any  time  undergo  specialization 
for  the  purpose  of  construction  or  destruction.  There  is  therefore 
no  basis  for  comparison  between  pathologic  conditions  of  bone  and 
enamel. 

The  Form  of  the  Structural  Elements. — The  enamel  is  made  up 
of  prismatic  rods  of  inorganic  matter,  held  together  by  an  inorganic 
cementing  substance.  All  other  calcified  tissues  are  made  up  of 
fibrous  intercellular  substance,  containing  inorganic  salts  and 
usually  arranged  in  layers. 

The  structure  of  the  enamel  differs  so  greatly  from  all  other 
calcified  tissues  that  it  is  difficult  to  compare  them  briefly. 


PLATE  III 


From  .1.  Howard  Mummery's 
"Mirrosropic  AtKilomy  of  the  Teeth." 


CHAPTER  IV. 
THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL. 

THE  enamel  is  composed  of  two  structural  elements: 

1.  The  enamel  rods  or  prisms,  sometimes  called  enamel  fibers. 

2.  The  interprismatic,  or  cementing  substance. 

Enamel  Rods. — The  enamel  rods  are  long,  slender,  prismatic 
rods  irregularly  five  or  six-sided1  and  alternately  expanded  and  con- 
stricted throughout  their  length  (Plate  III  and  Fig.  9).  They  are 
from  three  and  four-tenths  to  four  and  five-tenths  microns  in  diam- 
eter, and  many  of  them  extend  from  the  dento-enamel  junction  to 
the  surface  of  the  enamel.  They  are  of  the  same  diameter  at  their 
outer  and  inner  ends.  This  last  statement  is  emphasized,  as  the 
direct  opposite  is  stated  in  some  standard  text-books  of  histology. 
In  the  'formation  of  the  tissue  they  are  arranged  so  that  the  expan- 
sions in  adjoining  rods  come  opposite  to  each  other,  and  do  not 

1  This  statement  of  the  shape  of  the  enamel  prisms  must  be  taken  as  a  general 
statement,  just  as  columnar  epithelial  cells  are  described  as  five-sided  in  cross-section. 
In  the  enamel  prism,  as  in  the  epithelial  cell,  the  form  is  the  result  of  mutual  pressure, 
the  outlines  are  never  regular,  and  unevenness  in  the  distribution  of  the  pressure,  or 
lack  of  balance  in  direction  will  modify  the  form  of  the  prisms.  For  further  study 
of  the  form  and  relation  of  the  enamel  rods  the  student  is  referred  to  The  Microscopic 
Anatomy  of  the  Teeth,  by  J.  Howard  Mummery,  Chapter  II. 

DESCRIPTION  OF  PLATE  III. 
(From  J.  Howard  Mummery's  "Microscopic  Anatomy.") 

Drawings  from  teased  preparations  of  enamel  from  elephant,  except  Figs.  5,  6,  7  and 
8,  which  are  from  sections. 

FIG.  1. — Double-grooved  prisms  (elephant),  r,  ridges;  g,  grooves.  The  ridges  are 
often  seen  projecting  beyond  the  extremities  of  fragments. 

FIG.  2. — Single-grooved  prisms  (elephant),     r,  ridge;  g,  grooves. 

FIG.  3. — Two  double-grooved  prisms,  transverse  above  (elephant). 

FIG.  4. — Fragments  of  prisms  in  transverse  fracture  (elephant). 

FIG.  5. — Four  prisms  from  a  section  (elephant),  showing  surface  marking  and 
prominence  of  the  ridge  at  r. 

FIG.  6. — From  elephant:  bridges  in  transverse  section.  The  interprismatic  sub- 
stance appeared  dark  and  the  bridges  are  very  conspicuous  as  white  lines. 

FIG.  7. — Elephant.     From  a  section,  showing  a  wing  process  in  the  enamel. 

FIG.  8. — Elephant.  From  a  section,  showing  ridges  and  grooves,  r,  ridges;  g, 
grooves. 

FIG.  9. — Two  prisms  from  elephant,  showing  needle-splitting  (n)  and  intercolumnar 
bridges  (6). 

FIG.  10. — Fragment  of  elephant  enamel  in  transverse  section.  Two  entire  double, 
concave  prisms  are  seen  projecting,  with  feather  edges  and  intercolumnar  bridges  (b) . 

FIG.  11. — Fragments  of  prisms  seen  obliquely  (elephant). 

(41) 


42 


THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 


interlock  with  the  constrictions,  so  that  there  is  alternately  a  greater 
and  a  less  amount  of  cementing  substance  between  them. 

It  is  evident  that  the  outer  surface  of  the  enamel  is  much  greater 
than  the  surface  of  the  dentin  at  the  dento-enamel  junction.  This 
greater  area  is  obtained  in  two  ways: 

1.  The  rods  are  at  right  angles  to  the  dentin  at  the  dento-enamel 
junction,  but  are  seldom  at  right  angles  to  the  outer  surface.  This 
may  be  illustrated  by  bending  the  leaves  of  a  book,  or  cutting  a 
stack  of  paper  obliquely.  The  sheets  of  paper  are  of  the  same 
thickness,  but  when  cut  at  right  angles  to  the  sheets  the  area  of 
the  cut  surface  is  not  so  great  as  when  the  leaves  are  cut  diagonally. 


FIG.  9. — Enamel  rods  isolated  by  scraping.     (About  800  X) 

2.  Many  of  the  enamel  rods  undoubtedly  extend  from  the  dento- 
enamel  junction  to  the  surface  of  the  enamel,  though  it  is  difficult 
to  follow  individual  rods  through  this  distance,  but  there  are  also 
short  rods  which  extend  from  the  surface  part  way  to  the  dentin. 
These  short  rods  end  in  tapering  points  between  converging  rods 
that  extend  the  entire  distance.  The  short  rods  are  specially 
numerous  in  the  most  convex  portion  of  the  surface,  as  over  the  tips 
of  the  cusps,  occlusal  edges,  and  marginal  ridges.  These  areas 
therefore  become  of  special  importance  in  connection  with  the 
formation  of  enamel  walls,  as  will  be  considered  in  detail  later  on 
(Fig.  105). 


43 


Differences  between  Enamel  Rods  and  Cementing  Substance. — 
While  the  cementing  substance  and  the  substance  of  the  rods  are 
both  entirely  inorganic,  or,  more  correctly,  are  composed  entirely 
of  inorganic  salts,  they  differ  in  physical  and  chemical  properties 
as  follows: 

1.  The  cementing  substance  is  not  as  strong  as  the  prismatic 
substance. 

2.  The  cementing  substance  is  more  readily  soluble  in  dilute 
acids  than  the  rod  substance. 


FIG.  10. — Enamel  rods  in  thin  etched  section.     (About  800  X) 

3.  The  cementing  substance  is  of  slightly  different  (greater) 
refracting  index  than  the  substance  of  the  rod.  The  author  wishes 
to  emphasize  these  statements,  as  the  exact  opposite  is  found  in 
some  of  the  standard  texts,  at  least  concerning  the  first  and  second 
statements.  The  facts  are,  however,  so  easily  demonstrable  that 
anyone  may  satisfy  himself  without  difficulty. 

Relative  Strength  of  the  Enamel  Rods  and  the  Cementing  Substance. 
—The  cementing  substance  is  not  as  strong  as  the  substance  of 
the  rods.  The  most  striking  characteristics  of  the  enamel,  and  the 
first  to  attract  the  attention  of  the  student  and  the  operator,  are  its 
hardness  and  its  tendency  to  split  or  cleave  in  certain  directions. 


44          THE  STRUCTURAL  ELEMENTS  OF   THE  ENAMEL 

On  examination  it  is  found  that  this  is  determined  by  the  direction 
of  the  rods,  and  is  caused  by  the  difference  in  strength  between  the 
two  substances.  Sections  ground  at  right  angles  to  the  rod  direc- 
tion are  very  difficult  to  prepare  because  of  the  tendency  of  the 
section  to  break  to  pieces. 

If  a  section  that  is  beginning  to  crack  (Fig.  11)  is  studied,  the 
crack  is  found  to  follow  the  line  of  the  cementing  substance  running 
around  the  rods.  In  some  places  a  rod  may  be  split  through  its 
center,  but  most  of  the  rods  remain  perfect,  and  the  cementing  sub- 
stance breaks.  In  the  same  way  a  section  cut  in  the  direction  of  the 
rods  shows  the  crack  following  the  lines  of  the  cementing  substance 
(Fig.  12),  here  and  there  breaking  across  a  few  rods,  and  then  fol- 


FIG.  11. — Transverse  section  of  enamel  rods.     (About  80  X) 

lowing  the  direction  again;  but  the  rods  separate  on  the  line  of 
union,  not  at  the  centers  of  the  rods.  This  fact  becomes  fundamental 
in  the  cutting  of  enamel  and  in  the  preparation  of  strong  enamel 
walls. 

Relative  Solubility  of  Enamel  Rods  and  Cementing  Substance. — 
If  a  thin  section  of  enamel  cut  parallel  with  the  direction  of  the 
enamel  rods  is  mounted  in  water  and  hydrochloric  acid  (2  per  cent.) 
is  allowed  to  run  under  the  cover-glass  and  the  action  observed, 
it  will  be  seen  to  attack  the  cementing  substance  more  rapidly, 
dissolving  it  out  from  between  the  enamel  rods  and  attacking  their 
sides.  If  the  action  is  stopped  the  ends  of  the  rods  will  be  seen  pro- 
jecting like  the  pickets  of  a  fence,  as  shown  in  the  photograph 


RELATIVE  SOLUBILITY  OF  ENAMEL  RODS 


45 


(Fig.  13).  The  more  dilute  the  acid  the  greater  will  be  the  distance 
to  which  the  cementing  substance  is  removed  before  the  rods  are 
destroyed. 


FIG.  12. — Enamel  showing  direction  of  cleavage.     (About  70  X) 

Etching. — If  a  section  of  enamel  is  ground  at  right  angles  to  the 
direction  of  the  rods,  mounted  in  glycerin  and  photographed,  the 
outline  of  the  rods  will  be  seen  with  difficulty  (Fig.  14).  The  refract- 
ing index  of  the  two  substances  is  so  nearly  the  same  that  the  section 
seems  of  almost  uniform  transparency.  The  thinner  the  section, 


FIG.  13. — The  effect  of  acid  on  a  section  of  enamel. 

the  greater  will  be  the  difficulty  of  recognizing  the  rods.  Oblique 
illumination  and  the  use  of  a  small  diaphragm  will,  however,  resolve 
them.  If  the  section  is  washed  and  treated  with  2  per  cent,  hydro- 


46 


THE  STRUCTURAL  ELEMENTS  OF   THE  ENAMEL 


chloric  acid  for  a  few  seconds,  washed,  and  remounted  in  glycerin, 
the  rods  are  distinctly  outlined  (Fig.  15).  The  acid  attacks  the 
cementing  substance  and  the  surface  of  the  section  is  etched  as  if 
an  engraving  tool  had  been  run  around  the  rods.  The  fine  grooves 
on  the  surface  refract  the  light  and  outline  the  rods.  The  difference 
in  appearance  in  longitudinal  sections,  that  is,  sections  parallel  with 
the  direction  of  enamel  rods,  is  quite  as  striking.  For  the  study 


FIG.  14. — Enamel  ground  at  right  angles  to  the  rods.    Not  treated  with  acid. 

(About  500  X) 

of  enamel  rod  directions  this  etching  is  of  the  greatest  importance. 
Only  one  side  of  the  section  should  be  acted  upon  by  the  acid,  and 
the  section  should  be  mounted  etched  side  up.  If  etched  upon  both 
surfaces,  the  grooves  in  the  lower  surface  cannot  be  in  focus  at  the 
same  time  as  those  of  the  upper  surface  and  will  blur  the  definition. 
The  difference  in  the  solubility  of  the  rods  and  cementing  sub- 
stance is  beautifully  illustrated  in  the  effect  of  caries  on  the  structure 
of  the  enamel  and  caries  of  the  enamel  cannot  be  understood  unless 
these  fundamental  facts  are  remembered.  The  question,  "What 
causes  the  difference  in  solubility  between  the  enamel  rods  and 


RELATIVE  SOLUBILITY  OF  ENAMEL  RODS 


47 


the  cementing  substance?"  cannot  be  satisfactorily  answered  at 
the  present  time.  While  both  the  rods  and  the  cementing  substance 
are  normally  composed  entirely  of  inorganic  salts,  there  may  be 
different  salts  in  the  two  substances,  or  the  salts  may  be  in  different 
physical  condition.  There  is  great  need  for  careful  work  in  this 
field.  Recent  work  has  strongly  emphasized  the  distinctness  of 
the  two  structural  elements  of  the  enamel. 


FIG.  15. — The  same  section  as  Fig.  14  after  treatment  with  acid.     (About  500  X) 


First,  the  study  of  the  beginnings  of  caries  of  the  enamel,  and 
the  effect  of  caries  upon  the  structure  of  the  enamel,  brought  out 
the  difference  in  solubility  in  acids  and  showed  the  extent  of  tissue 
injury  before  a  cavity  is  formed.  Later,  the  study  of  hypoplasia 
developed  the  fact  that  certain  pathologic  or  abnormal  conditions 
may  hinder  or  entirely  prevent  the  formation  of  the  rods  while  the 
cementing  substance  is  formed,  and  still  more  recently  the  investi- 
gation of  dystrophies  of  the  enamel  occurring  in  certain  prescribed 
localities,  showed  perfect  rod  formation  and  entire  absence  of  the 
cementing  substance.  These  facts  suggest  the  hypothesis  that  the 


48          THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

enamel  rods  and  the  cementing  substance  have  a  different  origin, 
or  are  formed  by  different  cells,  and  that  pathological  conditions 
may  prevent  the  formation  of  one  and  not  the  other.  In  view  of 
these  factors  it  is  very  necessary  that  a  new  investigation  of  the 
process  of  enamel  formation  be  undertaken,  as  present  knowledge 
of  the  process  does  not  explain  such  conditions. 

Difference  in  Refracting  Index  between  the  Rods  and  the  Cementing 
Substance. — The  cementing  substance  is  of  slightly  greater  refract- 
ing index  than  the  substance  of  the  rods.  If  it  were  not  for  this  it 
would  be  impossible  to  see  the  rods  in  unetched  sections,  either 
longitudinal  or  transverse.  The  appearance  of  striation  seen  in 
longitudinal  sections  is  also  dependent  upon  this  difference  in  action 
on  transmitted  light. 

THE  EFFECT  OF  CARIES  ON  THE  STRUCTURE  OF  THE  ENAMEL. 

At  this  point  the  effect  of  caries  on  the  structure  of  the  enamel 
should  be  studied  as  a  demonstration  of  the  difference  in  solubility 
between  the  enamel  rods  and  the  interprismatic  substance. 

During  the  last  ten  years  of  his  life  the  work  of  the  late  Dr.  G.  V. 
Black  was  largely  devoted  to  the  study  of  the  beginning  of  caries  of 
the  enamel  and  the  extent  of  tissue  injury  before  an  actual  cavity  is 
produced.  This  has  placed  a  tremendous  emphasis  upon  the  value  for 
the  preservation  of  the  teeth,  of  the  treatment  of  caries  in  its  early 
rather  than  in  its  later  stages.  It  is  safe  to  say  that  if  caries  progresses 
until  a  patient  is  aware  of  a  cavity,  the  tooth  has  been  injured  more 
than  is  necessary  in  the  most  radical  treatment  of  the  same  cavity 
in  its  beginning  stages.  One  who  has  not  studied  carefully  the 
effect  of  caries  on  the  structure  of  the  enamel,  so  as  to  recognize 
the  extent  of  injury  to  the  structure  of  the  tissue  by  its  appearance 
to  the  naked  eye,  can  never  be  considered  fit  to  prepare  cavities 
as  a  treatment  for  the  disease.  The  beginnings  of  caries  must  be 
divided  into  two  classes:  (1)  Those  occurring  in  natural  defects 
of  structure;  (2)  those  beginning  upon  smooth  surfaces. 

Caries  Beginning  in  Natural  Defects  of  Structure. — These  are  the 
positions  in  which  caries  first  appears  and  in  which  it  presents  the 
greatest  intensity,  because  they  offer  ideal  conditions.  Such  open 
grooves  and  imperfectly  closed  pits  in  the  enamel  as  are  illus- 
trated in  Chapter  IX  become  filled  with  food  debris,  which 
furnish  ideal  culture  media  for  acid-forming  bacteria.  At  the 
opening  of  the  defect  the  acid  is  washed  away  by  the  saliva  as  fast 


THE  EFFECT  OF  CARIES  ON  THE  ENAMEL       49 

as  it  is  formed,  but  at  the  bottom  of  the  groove  it  is  confined  and 
acts  upon  the  enamel,  dissolving  out  the  cementing  substance 
from  between  the  rods  and  following  the  rod  direction  toward  the 
dento-enamel  junction.  The  form  of  the  disintegrated  tissue  in 
such  positions  is  always  that  of  a  cone  or  wedge,  with  the  apex 
at  the  opening  of  the  pit  or  groove  and  the  base  toward  the  dento- 
enamel  junction.  The  formation  of  acid  in  these  positions  is  often 
so  rapid  and  the  confinement  so  perfect  that  the  carious  process 
here  manifests  its  greatest  intensity,  the  action  often  dissolving 
the  rods  as  well  as  the  cementing  substance  and  progressing  across 
the  rods.  But  even  when  the  action  follows  the  rod  direction,  the 
form  will  be  broader  toward  the  dentin,  as  the  rods  are  inclined 


FIG.  16. — A  split  tooth,  showing  caries  beginning  in  an  occlusal  groove. 

toward  the  defect.  Figs.  16  and  17  show  split  teeth  illustrating 
the  disintegration  of  the  enamel  around  occlusal  defects.  The 
disintegration  area  appears  white  by  reflected  light  because  the 
cementing  substance  has  been  removed  from  between  the  rods  and 
the  resulting  air  spaces  refract  the  light.  As  soon  as  this  disinte- 
gration reaches  the  dento-enamel  junction,  the  acid  formed  passes 
through  the  now  porous  enamel  and  acts  much  more  rapidly  upon 
the  dentin.  Because  of  the  branching  of  the  dentinal  tubules  at 
the  dento-enamel  junction,  the  action  upon  the  dentin  spreads 
rapidly  along  this  line.  Soon  some  of  the  loosened  rods  between 
the  bottom  of  the  defect  and  the  dentin  are  either  entirely  dissolved 
or  displaced  or  dislodged,  and  the  microorganisms  are  admitted 
to  the  dentin.  The  decalcified  dentin  matrix  becomes  food  material 
4 


50 


THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 


for  the  bacteria,  and  the  space  produced  by  the  destruction  of 
tissue  furnishes  greater  space  for  decomposing  foodstuffs.  The 
acids  formed  attack  the  enamel  from  within  outward,  producing 


FIG.  17. — A  split  tooth,  showing  caries  progressing  in  an  occlusal  groove. 

what  has  been  called  backward  or  secondary  decay  of  enamel. 
At  the  mouth  of  the  defect  the  acid  is  still  washed  away,  and  there 
is  little  action  upon  the  tissue.  The  condition  progresses,  there- 
fore, until,  as  in  Fig.  18,  the  entire  occlusal  enamel  has  been  under- 
mined, and  all  of  the  undermined  area  has  been  greatly  weakened 


FIG.   18. — A  split  tooth,  showing  the  undermining  of  the  occlusal  enamel  by  caries 
spreading  at  the  dento-enamel  junction. 

by  the  solution  of  the  cementing  substance  from  between  the  rods. 
In  ground  sections  of  such  areas  as  shown  in  Fig.  21  the  disinte- 
grated area  appears  dark  by  transmitted  light.  Fig.  19  shows 


THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 


51 


the  progress  of  secondary  decay  from  an  occlusal  cavity.  In  this 
way  it  often  happens  that  the  entire  occlusal  enamel  is  destroyed 
before  the  original  defect  is  noticeably  enlarged. 

The  general  form  of  the  disintegrated  area  in  caries  beginning 
in  natural  defects  may  be  described  diagrammatically,  as  in  the 
enamel  a  cone  or  wedge  with  the  apex  toward  the  mouth  of  the 
defect  and  the  base  toward  the  dento-enamel  junction,  and  in  the 


FIG.  19. — A  section  showing  the  undermining  of  the  enamel  and  secondary  or 
backward  decay  at  1. 

dentin  a  cone  or  wedge  with  the  base  at  the  dento-enamel  junction 
and  the  apex  toward  the  pulp. 

Caries  Beginning  on  Smooth  Surfaces. — Caries  upon  smooth  sur- 
faces of  the  enamel  is  always  due  to  the  growth  of  a  colony  of 
bacteria  which  becomes  attached  to  the  surface  by  the  formation 
of  material,  causing  them  to  adhere  to  the  surface  and  at  the  same 
time  confining  their  acid  products  in  contact  with  the  enamel 


52          THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

preventing  its  dissipation  in  the  saliva  and  allowing  it  to  combine 
with  the  inorganic  salts  of  the  tissue  elements.  This  is  not  the 
place  to  consider  the  bacteriology  of  caries,  but  the  effect  upon 
the  structure  of  the  enamel  cannot  be  understood  without  a  clear 
conception  of  the  microbic  plaques.  A  growth  of  masses  of  micro- 
organisms upon  the  surface  of  a  tooth  does  not  constitute  a  plaque. 
Many  very  filthy  mouths  are  found  where  most  of  the  surfaces  of 
the  teeth  are  covered  by  thick,  furry  masses,  and  where  there  is 
little  or  no  attack  of  the  enamel.  Either  acid  is  not  formed  or  it 
is  at  once  lost  by  solution  in  the  saliva.  Caries  shows  the  greatest 
intensity  in  comparatively  clean  mouths,  in  which  something  in 
the  nature  of  the  saliva  causes  the  bacteria  to  produce  a  tough 
zooglea,  which  attaches  them  to  the  tooth  surface  and  confines 
the  products  of  their  activity.  This  zooglea  presents  some  of  the 
phenomena  of  a  dialyzing  membrane.  Through  it  the  micro- 
organisms receive  their  food  materials,  and  their  products  are 
neutralized  by  chemical  action  on  the  surface  upon  which  the 
colony  is  growing.  Colonies  lodge  in  the  most  favorable  spots 
and  extend  from  these  points  into  areas  that  are  less  liable  to  main- 
tain their  attachment.  The  more  perfect  the  confinement  of  the 
acid,  and  the  more  rapid  the  rate  of  its  formation,  the  greater  will 
be  the  intensity  of  the  destructive  process.  The  more  easily  the 
colony  is  able  to  maintain  itself  in  its  position  and  extend  upon 
the  surface,  the  greater  is  the  liability.  As  the  colony  becomes 
thickest  at  the  point  of  beginning,  it  is  evident  that  the  most  acid 
is  formed  here,  and  it  is  therefore  the  point  of  greatest  intensity. 
It  is  also  the  point  at  which  the  growth  began,  and  therefore 
the  spot  where  the  action  on  the  tissue  has  been  longest  in  opera- 
tion. It  is  also  apparent  that  there  may  be  great  intensity  with 
limited  liability,  and  great  liability  with  very  low  intensity,  and 
the  effect  upon  the  tissue  will  be  different  in  the  two  cases. 

The  appearance  of  the  tissue  becomes  an  index  for  estimating 
the  intensity  and  liability  in  a  given  case.  The  character  of  the 
effect  of  the  disease  on  the  appearance  of  the  enamel,  as  well  as  the 
direction  of  the  extension  upon  the  surface  of  the  tooth,  become 
most  important  factors  in  the  diagnosis  of  any  case,  and  the  diag- 
nosis is  the  basis  for  the  treatment  required.  The  increased  appre- 
ciation of  the  extent  of  disintegration  of  the  enamel  before  an 
actual  cavity  is  apparent  in  a  tooth  has  been  one  of  the  most 
important  results  of  Dr.  Black's  study  of  caries  of  the  enamel. 
The  author  has  been  intimately  associated  with  this  work,  and  has 


THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 


53 


been  amazed  at  the  extent  and  character  of  the  effect  of  caries 
upon  the  structure  of  the  enamel  in  what  may  be  called  the  early 
stages  of  the  disease. 

Progress  of  Caries. — A  colony  of  bacteria  becomes  attached 
to  the  proximal  surface  of  an  incisor  just  to  the  gingival  of  the 
contact  point,  and  remains  there  some  time.  If  the  surface  of 
the  tooth  can  then  be  examined,  a  white  spot  will  be  seen  at  Fig. 
21;  the  area  appears  white  because  the  cementing  substance  has 
been  removed  from  between  the  enamel  rods,  as  will  be  seen  later, 
and  the  air  that  occupies  the  spaces  diffuses  the  light.  If  a  tooth 
is  split  through  such  a  spot  and  viewed  from  the  surface,  the  appear- 
ance will  be  as  shown  in  Fig.  20.  If  a  section  were  ground  through 
the  spot  and  the  tissue  preserved,  the  ends  of  the  enamel  rods 


FIG.  20 


FIG.  21 


21. 


FIG.   20.  —  A  split  tooth  cut  through 
a  white  spot  as  is  shown  in  Fig. 


FIG.  21. — A  superior  central  incisor, 
showing  a  white  spot  just  to  the  gingival 
of  the  contact  point. 


would  be  seen  pointed  and  projecting  like  the  pickets  of  a  fence, 
giving  the  same  appearance  as  that  produced  by  the  action  of 
acid  upon  a  ground  section,  as  illustrated  in  Fig.  15. 

The  surface  of  the  enamel  is  therefore  no  longer  smooth,  but 
roughened.  The  roughness  may  often  be  felt  by  passing  a  very 
fine-pointed  steel  explorer  over  the  surface.  If  the  colony  be 
dislodged  at  this  stage  it  is  evident  that  it  is  much  easier  for  a  new 
one  to  become  attached.  These  whitened  areas  are  often  invisible 
unless  the  tissue  is  dried,  because  the  saliva  fills  the  spaces.  If  the 
surface  is  dried  the  refraction  of  the  light  by  the  air  whitens  the 
affected  area. 

A  good  comparison  is  furnished  in  a  very  familiar  phenomenon. 
Snow  is  white  because  the  air  and  the  microscopic  ice  crystals 


54 


THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 


are  of  different  refracting  index,  and  the  light  is  diffused  by  passing 
from  air  and  ice  crystals.  If  a  snowball  is  saturated  with  water 
it  loses  its  whiteness  and  becomes  translucent,  because  the  water, 
which  is  nearly  of  the  same  refracting  index  as  ice,  fills  the  spaces 
between  the  ice  crystals,  and  the  light  is  not  diffused.  If  the  white 
area  of  such  a  tooth  is  split  through  the  center  with  an  aluminum 
disk  charged  with  emery  powder,  the  enamel  rods  will  be  found 


FIG.  22. — A  thin  section  of  carious  enamel  ground  on  the  cover-glass  with  balsam: 
E,  sound  enamel;  X,  carious  enamel  in  which  the  cementing  substance  had  been 
dissolved  from  between  the  rods. 

entirely  separated  by  the  solution  of  the  cementing  substance, 
and  the  cross-striation  will  be  much  more  apparent  because  the 
unevenness  in  the  diameter  of  the  rods  has  been  increased  by  the 
action  of  the  acid. 

Formerly  it  was  impossible  to  grind  a  section  through  such  a 
spot  and  preserve  the  tissue.  Until  methods  were  devised  by  Dr. 
Black,  it  was  impossible  to  preserve  the  tissue  and  examine  its 


THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 


55 


condition.  These  methods  demonstrate  definitely  that  in  the  dis- 
integrated area  the  cementing  substance  is  dissolved  in  large  areas 
before  any  of  the  rods  are  dissolved  or  destroyed.  The  first  sections 


FIG.  23. — Carious  enamel  ground  on  the  cover-glass  by  the  shellac  method.  In 
the  region  X  the  cementing  substance  dissolved  from  between  the  rods  has  been 
replaced  by  shellac. 


of  such  areas  were  obtained  by  polishing  the  surfaces  and  cementing 
the  split  tooth  to  the  cover-glass  with  balsam,  completing  the  grind- 
ing and  mounting  without  loosening  the  section.  In  this  way  the 


56          THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

spaces  between  the  rods  were  filled  with  balsam  and  so  were  held  in 
place.  Fig.  22  shows  a  photograph  of  a  section  made  in  this  way,  and 
the  spaces  between  the  rods  and  the  distinct  cross-striation  are  seen. 
Later  it  was  found  that  by  dehydrating  and  immersing  in  a  solu- 
tion of  brown  shellac,  the  shellac  could  be  made  to  take  the  place 
of  the  lost  cementing  substance,  then  the  polished  surface  of  the 
sawed-out  section  could  be  fastened  to  the  cover-glass  with  shellac, 
and  the  specimen  handled  more  easily.  Fig.  23  shows  a  photo- 
graph of  carious  enamel  made  in  this  way.  The  rods  are  preserved 
in  place  and  the  dark  shellac  marks  the  disintegrated  area  very 
clearly. 

Stages  in  the  Progress  of  Caries. — The  progress  of  caries  on  smooth 
surfaces  of  the  enamel  may  be  divided  into  three  periods,  according 
to  its  effect  upon  the  structure  of  the  tissue. 

1.  From  the  lodgment  of  the  colony  until  the  action  reaches 
the  dento-enamel  junction. 

2.  From  the  reaching  of  the  dento-enamel  junction  until  the 
rods  begin  to  fall  out. 

3.  After  a  cavity  is  produced. 

First  Period. — The  form  of  the  disintegrated  tissue  in  the  first 
period  is  always  that  of  an  irregular  cone.  Its  base  is  on  the  sur- 
face of  the  enamel,  its  outline  is  the  boundary  of  the  colony,  and 
the  apex  is  toward  the  dentin  in  the  direction  of  the  enamel  rods 
from  the  starting-point  of  the  colony.  The  inner  boundary  of  the 
area  is  never  even,  but  shows  flame-like  extensions  toward  the 
dentin  in  the  direction  of  the  rods.  This  is  more  marked  in  some 
cases  than  in  others,  and  sometimes  suggests  that  the  presence  of 
a  colony  on  the  surface  has  been  intermittent  (Plates  IV,  V,  VI). 

The  boundary  between  the  perfect  and  the  disintegrated  area 
is  usually  marked  by  a  darker  area,  the  significance  of  which  is 
not  now  understood.  If  the  disease  progresses  continuously  the 
affected  tissue  always  appears  white  by  reflected  light,  but  if  the 
progress  has  been  intermittent,  especially  if  there  have  been  con- 
siderable periods  in  which  no  colony  has  been  attached  to  the  sur- 
face, the  area  darkens,  becoming  brownish  or  almost  black.  This 
is  produced  by  organic  materials  filling  the  space  between  the 
enamel  rods  and  decomposing,  with  the  probable  formation  of  sul- 
phides of  dark  color  in  the  spaces.  If  immunity  to  caries  is  attained 
before  the  effect  upon  the  tissue  has  penetrated  to  the  dento-enamel 
junction,  this  will  occur,  and  the  spot  changes  from  a  white  to  a 
brownish  or  black  color.  Such  spots  will  be  found  in  some  places 


PLATE  IV 


A  Section  through   a  Carious  Spot  in  the   First  Period. 

Showing  extension    of  the  attack  on  the   surface   toward  the   gimnval. 


PLATE  VI 


A  Section  through  a  Carious  Spot  in  the  Second   Period. 

X,  disintegrated  enamel  at  the  point  of  first  lodgment  of  the  colony; 
Z,  disintegrated  enamel  as  the  result  of  the  extension  of  the  colony  on  the 
surface  toward  the  occlusal;  E,  sound  enamel;  D,  clentin. 


THE  EFFECT  OF  CARIES  ON  THE  ENAMEL 


57 


on  most  teeth  extracted  from  immune  persons.    Work  of  Dr.  Miller 
has  indicated  that  such  spots  are  more  resistant  to  the  progress 


FIG.  24. — A  section   through  a   white  spot  in  the  first  period  of  attack:    X,  disinte- 
grated enamel;   E  sound  enamel;   D,  dentin. 


58 


THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 


of  caries  than  perfect  enamel  surfaces.  At  any  time  during  the 
first  period,  therefore,  the  destruction  may  be  arrested  by  the  com- 
ing of  immunity,  which  prevents  the  attachment  of  colonies  to 
the  tooth  surface  by  the  formation  of  plaques. 


FIG.  25. — A  section  through  a  carious  spot  in  the  first  period.  The  attack  has 
apparently  been  slow  and  intermittent:  X,  disintegrated  enamel;  E,  sound  enamel; 
D,  dentin. 

Second  Period. — This  period  extends  from  the  time  when  the 
action  of  the  acid  reaches  the  dento-enamel  junction  until  the 
rods  are  destroyed  or  fall  out.  As  soon  as  the  solution  of  the 
cementing  substance  reaches  the  dento-enamel  junction  at  the 
point  of  the  advancing  cone,  the  solution  of  the  inorganic  salts 


THE  EFFECT  OF  CARIES  ON  THE  ENAMEL  59 

from  the  dentin  matrix  begins.     It  must  be  remembered  that 
the  acid  is  formed  by  the  microorganisms  on  the  surface  of  the 


FIG.  26. — A  section  through  a  carious  spot  in  the  first  period,  showing  the  flame- 
like  projections  toward  the  dentin:  A',  disintegrated  enamel;  E,  sound  enamel; 
D,  dentin. 


60         THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

enamel,  and  filters  through  the  spaces  between  the  enamel  rods. 
The  decalcification  of  the  dentin  may  be  considerable,  while  the 
surface  of  the  enamel  is  still  preserved.  In  this  period  the  swelling 
of  the  surface  is  always  noticeable.  This  results  in  increasing  the 
area  of  the  contact  and  therefore  allowing  the  colony  to  extend 
its  limits,  increasing  the  extent  of  the  surface  attack.  This  is 
especially  noticeable  toward  the  gingival,  and  is  shown  in  Plate  IV, 
which  is,  however,  shown  in  the  first  period  of  caries.  In  the 
disintegrated  area  in  this  stage,  as  wTell  as  in  the  first  stage,  the 
diameter  of  the  enamel  rods  is  always  considerably  reduced  and  the 
striation  rendered  more  apparent.  In  caries  of  great  intensity 


FIG.  27.  —  A  tooth  split  through  a  spot,  FIG.  28. — A  tooth  split  through 

showing      great     intensity     but     low     lia-          spots,  showing  low  intensity  but 
bility.  great  liability. 

but  low  liability  the  reduction  in  the  diameter  of  the  enamel  rods 
is  rapid,  and  they  are  soon  destroyed,  while  the  area  of  the  surface 
attacked  is  small  (Fig.  27). 

In  caries  of  low  intensity  but  great  liability  the  diameter  of  the 
rods  is  slowly  reduced,  while  the  area  of  surface  attacked,  and 
consequently  the  area  of  disintegration,  is  large  (Fig.  28).  These 
conditions  should  be  studied  in  the  macroscopic  appearance  of 
caries  at  the  chair. 

The  decalcified  dentin  matrix  shrinks  and  more  or  less  of  a 
space  is  formed  under  the  enamel. 


THE  EFFECT  OF  CARIES  ON   THE  ENAMEL 


61 


The  action  of  the  acid  follows  the  tubules  of  the  dentin  toward 
the  pulp,  and  spreads  through  their  branches  laterally  near  the 
dento-enamel  junction  so  that  the  form  of  the  disintegrated  dentin 
is  always  that  of  a  cone,  with  the  base  at  the  dento-enamel  junction 
and  the  apex  toward  the  pulp  chamber.  It  is  important,  however, 
to  remember  that  in  this  stage  no  microorganisms  have  entered 
the  tissue,  and  the  effect  upon  it  is  the  result  of  the  action  of  sub- 
stances formed  upon  the  surface.  The  extent  of  enamel  disintegra- 
tion and  decalcification  of  dentin,  in  this  stage,  is  much  greater 
than  anyone  supposed  before  such  specimens  as  the  present  illus- 
trations were  made. 


FIG.  29.— A  drawing  showing  the  microorganisms  of  caries  growing  through  the 
dentinal  tubules.     (G.  V.  Black.) 

Third  Period. — This  embraces  the  period  after  the  enamel  rods 
have  begun  to  fall  out  and  an  actual  cavity  is  apparent.  As  soon 
as  this  occurs  the  surface  of  the  tooth  at  the  point  where  the  forma- 
tion of  the  colony  began  is  destroyed  and  the  protected  point  is 
lost,  and  the  extension  of  surface  attack  ceases.  The  microorgan- 
isms are  admitted  to  the  dentin,  where  they  grow  through  the 
dentinal  tubules,  spreading  rapidly  at  the  dento-enamel  junction 
(Fig.  29).  The  dentin  is  always  decalcified  in  advance  of  the 
penetration  of  the  microorganisms.  The  acid  formed  within  the 
cavity  attacks  the  cementing  substance  between  the  enamel  rods, 


62         THE  STRUCTURAL  ELEMENTS  OF  THE  ENAMEL 

and  proceeds  from  the  dento-enamel  junction  outward.  This 
is  called  secondary  or  backward  decay  of  the  enamel,  and  as  a 
result  of  it,  large  areas  are  disintegrated  until  they  are  sufficiently 
weakened  to  break  into  the  cavity.  This  condition  is  shown  in 
Fig.  19,  in  which  the  area  indicated  by  1  has  had  the  cementing 
substance  entirely  removed  from  between  the  rods,  and  is  in  the 
same  structural  condition  as  the  disintegrated  areas  in  the  first 
and  second  stage.  It  is  safe  to  say  that  in  the  past  few  cavities 
have  been  filled  until  the  enamel  has  caved  in.  It  is  equally  certain 
that  in  a  large  proportion  of  cases,  by  the  time  this  has  happened, 
the  removal  of  all  disintegrated  tissue  will  require  a  greater  loss 
of  tooth  substance  than  would  be  required  for  the  prevention  of 
a  new  surface  attack,  at  the  margin  of  the  filling,  if  the  case  had 
been  treated  as  a  beginning  instead  of  a  burrowing  decay. 


CHAPTER  V. 
CHARACTERISTICS  OF  THE  ENAMEL  TISSUE. 

FROM  what  has  been  said  of  the  structural  elements  of  the  tissue, 
their  physical  and  chemical  properties,  and  their  arrangement 
in  the  tissue,  it  is  apparent  that  the  striking  characteristics  of  the 
enamel  are  the  result  of  these  factors;  and  that  it  can  be  intelli- 
gently dealt  with  only  by  thinking  of  it  always  in  these  terms. 


FIG.  30. — Enamel  showing  cleavage. 

The  enamel  may  be  crudely  compared  to  a  pavement  made 
up  of  tall  columns  closely  cemented  together  by  an  inorganic 
cement.  The  wear  comes  on  the  ends  of  the  columns,  and  they 
furnish  great  resistance  to  the  abrasion  of  friction.  When  sup- 
ported upon  a  good  and  elastic  foundation  it  is  very  difficult  to 
break  it  down,  but  when  an  opening  has  been  made  in  it,  and  the 
foundation  removed  from  underneath,  the  columns  are  com- 
paratively easily  split  off  and  tumbled  into  the  opening  (Fig.  30). 
This  figure  is  crude,  but  it  is  a  very  helpful  one  in  learning  to  think 
of  the  enamel  in  terms  of  its  structural  elements. 

(63) 


64 


CHARACTERISTICS  OF   THE  ENAMEL  TISSUE 


Straight  Enamel. — Upon  the  axial  surfaces  of  the  teeth  the  rods 
are  usually  straight  and  parallel  with  each  other,  and  most  of 
split  extend  from  the  dentin  to  the  surface.  Such  enamel  will 
split  or  cleave  in  the  direction  of  the  rods  with  comparative  ease, 
and  breaks  down  very  readily  when  the  dentin  is  removed  from 

under  it.  It  will  usually  cleave  through 
its  entire  thickness  and  break  away  from 
sound  dentin  when  properly  attacked 
with  sharp  hand  instruments.  Such 
enamel  is  called  straight  enamel,  as 
contrasted  with  gnarled  enamel.  It  is 
best  illustrated  by  cutting  sections 
labiolingually  through  the  incisors, 
though  there  is  considerable  variation 
in  different  teeth  (Figs.  12  and  31). 

Gnarled  Enamel. — Upon  the  occlusal 
surfaces  of  the  molars  and  bicuspids, 
and  especially  over  the  tips  of  cusps 
and  marginal  ridges,  the  rods  are  seldom 
straight  and  parallel  through  the  thick- 
ness of  the  enamel,  but  are  wound  and 
twisted  about  each  other,  especially  in 
the  deeper  half  toward  the  dento-enamel 
junction.  This  is  known  as  gnarled 
enamel,  and  its  appearance  is  in  marked 
contrast  with  straight  enamel. 

Toward  the  surface  the  rods  are 
usually  straight  and  parallel  for  a 
longer  or  shorter  distance,  but  as  the 
dento-enamel  junction  is  approached 
they  become  twisted.  This  is  true  of 
most  of  the  occlusal  surfaces  of  molars 
and  bicuspids,  but  the  gnarled  condition 
extends  farther  toward  the  surface 
over  the  tips  of  the  cusps,  or  the  point 
at  which  the  rods  were  first  completed 

in  the  growth  of  the  crown.  As  cleavage  is  caused  by  the  difference 
in  the  strength  of  the  rods  and  cementing  substance,  it  is  easy  to  see 
that  gnarled  enamel  will  not  split  or  cleave  easily  when  resting  upon 
sound  dentin.  This  is  often  encountered  in  extending  occlusal 
cavities.  The  straight  portion  will  split,  but  where  the  rods  begin  to 


FIG.  31. — Straight  enamel 
rods. 


EFFECT  OF  STRUCTURE  ON  THE  CUTTING  OF  ENAMEL      65 

twist  they  break  off,  leaving  a  portion  resting  on  the  dentin  which 
will  resist  the  attack  of  any  cutting  instrument  from  the  surface 
(Figs.  32,  33,  and  34). 


FIG.  32.— Gnarled  enamel.     (About  80  X) 

The  Effect  of  Structure  on  the  Cutting  of  Enamel. — The  two  kinds 
of  enamel  may  be  compared  to  straight-grained  pine  wood  and  a 
5 


GG 


CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 


pine  knot.  The  first  will  split  easily  in  the  direction  of  the  fiber, 
the  latter  will  split  only  in  an  irregular  way  and  with  the  greatest 
difficulty.  This  difference 
in  the  arrangement  of  the 
structural  elements  leads  to 
the  difference  in  the  feeling 
of  various  teeth  to  cutting 
instruments,  and  is  the 
basis  for  the  clinical  ex- 
perience of  hard  and  soft 
teeth.  It  is  not  a  matter  of 
degree  of  calcification,  but 


FIG.  33. — Gnarled  enamel. 


FIG.  34. — Gnarled  enamel  from  etched  section 
(About  100  X) 


the  arrangement  of  the  structural  elements,  and  gnarled  enamel 
will  break  down  as  rapidly  under  the  effect  of  caries  as  will 
straight  enamel. 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL  67 

From  a  study  of  the  positions  in  which  the  rods  are  usually 
twisted  about  each  other,  and  those  in  which  they  are  usually 
straight,  it  seems  probable  that  the  twisting  is  due  to  movements 
in  the  dental  papilla  and  the  enamel  organ  during  the  formation 
of  the  tissue.  These  movements  may  be  produced  by  variations 
in  the  blood-pressure  which  cause  oscillations,  or  shiftings  of  the 
tissues  on  each  other.  These  differences  in  the  arrangement  of 
the  structural  elements  of  the  enamel  must  be  constantly  kept  in 
mind,  and  will  be  referred  to  many  times  in  connection  with  the 
use  of  cutting  instruments  on  the  enamel  and  the  preparation  of 
cavity  wyalls. 

APPEARANCES   CHARACTERISTIC   OF  ENAMEL. 

Striation. — Striation  is  the  appearance  of  fine  light  and  dark 
markings  occurring  alternately  in  the  length  of  the  enamel  rods. 
This  is  not  unlike  the  striation  of  voluntary  muscle  fibers,  and  has 
a  similar  cause.  It  is  seen  both  in  thin  sections  cut  in  the  direction 
of  the  rods,  and  in  isolated  enamel  rods.  It  is  caused  by  the  alter- 
nate expansions  and  constrictions  of  the  rods  and  the  difference 
in  the  refracting  index  between  the  rods  and  the  cementing  sub- 
stance. 

If  isolated  rods  (Fig.  35)  are  observed  with  a  |  or  yV  objective, 
they  will  be  seen  to  be  marked  by  alternate  light  and  dark  areas 
across  the  rods;  on  changing  the  focus  up  and  down,  the  light 
and  dark  areas  will  change  places,  just  as  in  looking  at  a  red  blood 
corpuscle  the  center  may  appear  dark  and  the  rim  light,  or  the 
center  light  and  the  rim  dark,  depending  upon  the  exactness  of 
focus.  This  is  caused  by  the  refraction  of  the  light  as  it  passes 
through  the  convex  and  concave  portions  of  the  rod.  If  the 
cementing  substance  were  of  exactly  the  same  refracting  index 
as  the  rods,  when  the  rods  were  fastened  together  in  the  tissue 
there  would  be  no  appearance  of  striation,  but  as  it  is  not,  refrac- 
tion of  light  occurs  in  passing  from  rod  substance  to  cementing 
substance,  and  the  striation  is  apparent  in  sections.  There  is  con- 
siderable difference  in  the  distinctness  of  striation  in  different 
sections  of  enamel.  This  is  probably  due  to  the  fact  that  the 
cementing  substance  has  more  nearly  the  same  refracting  index 
as  the  rods  in  some  specimens.  "When  the  formation  of  enamel 
has  been  studied  it  will  be^found^that^the  enamel  rods  have  been 
formed  by  globules  which  are  deposited  one  on  top  of  the  other 


68 


CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 


to  form  the  rods,  and  the  cementing  substance  fills  up  the  space. 
The  globules  in  the  adjacent  rods  come  opposite  each  other,  so 
that  there  is  alternately  a  greater  and  a  less  amount  of  cementing 
substance  between  the  rods.  Each  cross-mark  therefore  repre- 
sents a  globule  deposited  in  the  formation  of  the  rod,  and  striation 
may  be  said  to  be  a  record  of  the  growth  of  the  individual  rods 
(Figs.  36  and  37). 

Imperfections  in  the  cementing  substance  render  the  striation 
more  apparent  because  they  increase  the  difference  in  refraction 


FIG.  35. — Isolated  enamel  rods.     (About  1000  X) 

between  the  two  substances.  The  action  of  acid  either  upon 
isolated  rods  or  upon  sections  renders  striation  more  apparent 
because  it  attacks  the  cementing  substance  faster  than  the  globules 
forming  the  rods,  and  therefore  increases  the  refraction.  Von 
Beber  has  claimed  that  the  appearance  of  striation  was  caused 
by  the  action  of  acid  on  the  section,  and  that  even  in  mounting 
in  balsam  the  acidity  of  the  balsam  affected  the  tissue.  It  is  true 
that  any  action  of  acid  increases  the  distinctness  of  the  cross- 
striation,  but  it  is  not  the  cause  of  it. 

Stratification,  or  the  Bands  of  Retzius. — If  longitudinal  sections 
of  moderate  thickness  are  observed  with  the  low  power,  brownish 
bands  are  seen  running  through  the  enamel,  which  suggests  the 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL  69 

appearance  of  stratification  in  rocks.  These  were  first  described 
by  Retzius  and  were  named  after  him — the  brown  bands  or  striae 
of  Retzius.  A  better  name  would  be  incremental  lines. 


FIG.  36. — Enamel  showing  both  striation  and  stratification.     (About  80  X) 


FIG.  37. — Enamel  showing  striation.    (About  1000  X) 


70 


CHARACTERISTICS  OF  THE  ENAMEL   TISSUE 


The  bands  of  Retzius,  or  incremental  lines,  are  caused  by  actual 
coloring  matter  which  is  deposited  with  the  inorganic  salts  in  the 


FIG.  38. — -Tip  of  an  incisor.     (About  50  X) 

formation  of  the  tissue.  They  are  therefore  best  seen  with  low 
powers  and  in  sections  that  are  not  too  thin.  In  sections  that  are 
thinner  than  the  diameter  of  a  single  rod,  or  less  than  four  microns, 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL 


71 


they  become  almost  invisible.  For  the  study  of  the  bands  of  Retzius 
sections  should  be  ground  labiolingually  through  the  incisors, 
buccolingually  through  the  bicuspids  and  molars,  striking  the 
center  of  the  cusps.  They  may  be  studied  also  in  mesiodistal 
sections,  but  the  sections  should  be  in  such  a  direction  as  to  be  at 


FIG.  39. — Incisor  tip  showing  stratification  or  incremental  lines.     Rods  at  A  were 
fully  formed  at  the  time  the  rods  at  B  were  beginning  to  form.     (About  50  X) 

right  angles  to  the  zones.  Fig.  38  shows  the  tip  of  an  incisor  in 
which  the  bands  are  very  well  marked.  They  are  seen  to  begin  at 
the  dento-enamel  junction  on  the  incisal  edge,  and  sweep  in  larger 
and  larger  zones  around  this  point.  Each  band  represents  what 
was  at  one  time  the  surface  of  the  enamel  already  formed,  and  the 
line  upon  which  formation  was  progressing.  They  are  therefore 


72 


CHARACTERISTICS  OF  THE  ENAMEL  TISSUE 


truly  incremental  lines.  The  zones  reach  the  surface  of  the  enamel 
first  at  the  point  over  the  center  of  beginning  calcification,  and 
the  succeeding  bands  extend  from  the  surface  of  the  enamel,  near 
the  occlusal,  to  the  dento-enamel  junction  much  farther  apically, 


FIG.  40. — Stratification  of  enamel;  the  cusp  of  a  bicuspid:  De,  dento-enamel 
junction;  Ed,  enamel  defect  showing  in  the  heavy  stratification  band;  Ig,  inter- 
globular  spaces  in  the  dentin.  (About  40  X) 


and  corresponding  lines  are  seen  on  opposite  sides  of  the  section. 
In  Fig.  39,  the  band  which  is  at  the  surface  at  A  and  A'  reaches  the 
dento-enamel  junction  at  B  and  B'.  This  means  that  when  the 
enamel  rods  which  form  the  surface  at  A  were  completed,  the  rods 


APPEARANCES  CHARACTERISTIC  OF  ENAMEL 


73 


at  B  were  just  beginning  to  be  formed  at  the  dento-enamel  junc- 
tion. A  layer  of  functioning  ameloblasts  occupied  this  position. 
The  bands  of  Retzius  are  always  curved  and  usually  pass  obliquely 
across  the  enamel  rods,  but  are  parallel  neither  with  the  dento- 
enamel  junction  or  the  surface  of  the  enamel.  As  they  pass  toward 
the  gingival  the  angle  which  they  form  with  the  axis  of  the  tooth 
becomes  greater.  Any  disturbance  of  nutrition  which  affects  the 
formation  of  enamel  is  always  shown  in  the  increased  distinctness 
of  the  bands  (Fig.  40). 

The  bands  of  Retzius  therefore  form  a  record  of  the  formation 
of  the  tissue,  and  by  their  study  the  points  of  beginning  calcifi- 


FIG.  41. — Lines  of  Schreger.     (About  5  X) 


cation  and  the  manner  of  the  development  of  the  tooth  crown  may 
be  followed.  This  will  be  considered  again  in  connection  with  the 
grooves,  pits,  and  natural  defects  of  enamel. 

Lines  of  Schreger. — These  are  lines  appearing  in  the  enamel 
extending  from  the  dento-enamel  junction  to  or  toward  the  sur- 
face. They  are  caused  by  the  direction  in  wrhich  the  enamel  rods 
are  cut.  They  may  be  seen  in  sections,  but  are  best  shown  by 
photographing  the  cut  surface  of  the  enamel  by  reflected  light  and 
with  very  low  magnification.  The  rods  are  twisting  about  each 
other,  and  in  one  streak  they  are  cut  longitudinally,  in  the  next 
obliquely,  and  the  alternations  of  these  directions  cause  the  appear- 
ance of  the  lines  (Fig.  41). 


74 


Nasmyth's  Membrane  (The  Enamel  Cuticle.) — There  has  been  a 
vast  amount  of  writing  and  fruitless  speculation  in  regard  to  this 
structure.  The  facts  which  have  led  to  all  this  speculation  can  be 
simply  stated.  If  a  freshly  extracted  tooth,  that  has  not  been 
exposed  to  wear,  is  decalcified  or  treated  with  dilute  nitric  acid 


(other  acids  may  be  used)  a  membrane  can  be  floated  from  its 
surface  which  is  found  to  be  made  up  of  two  layers.  (1)  A  clear 
structureless  layer  which  was  in  contact  with  the  surface  of  the 
enamel  and  bears  the  imprints  of  the  ends  of  the  enamel  rods  on  its 
surface.  (2)  An  outer  cellular  layer  made  up  of  a  layer  or  layers  of 
epithelial  cells.  Unfortunately  the  study  of  Nasmyth's  membrane 
seems  to  have  been  made  from  extracted  teeth  and  not  from  sections 
which  retained  the  teeth  and  all  of  the  supporting  tissues  in  relation. 
Two  distinct  explanations  have  been  given  to  this  structure: 


75 


(1)  Owen  and  Tomes  considered  it  as  not  epithelial  but  a  deposit 
of  coronal  cementum  on  the  surface  of  the  enamel  before  the  erup- 
tion of  the  tooth  as  occurs  in  the  teeth  of  ungulates.  (2)  Huxley, 
Lent,  Kolliker,  Waldyer,  Paul,  Mummery  and  others  have  recog- 
nized its  epithelial  origin  and  described  its  structure  in  detail.  All 


FIG.  43 

have  considered  it  as  in  some  way  related  in  origin  to  the  enamel 
organ  but  as  to  the  way  in  which  it  is  formed  or  the  nature  of  the 
relationship  there  is  no  agreement. 

In  the  opinion  of  the  writer,  coronal  cementum  occurs  on  the 
enamel  surface  and  in  the  grooves  of  the  crowns  of  many  human 


76  CHARACTERISTICS  OF   THE  ENAMEL  TISSUE 

teeth  but  is  in  no  way  related  to  the  structure  described  as  Nas- 
myth's  Membrane.  On  the  other  hand,  while  the  structure  is 
undoubtedly  of  epithelial  character,  he  does  not  believe  that  it  is 
related  to  the  enamel  organ  or  the  formative  epithelium  of  the 
enamel  in  origin. 

On  the  eruption  of  the  tooth  the  epithelium  of  the  gingival  fold, 
at  least  on  the  deeper  portions  is  held  firmly  against  the  surface  of 
the  enamel  by  the  pressure  of  the  surrounding  tissues,  and  the 
surface  cells  are  quite  firmly  adherent  to  the  surface  of  the  enamel. 
The  multiplication  of  epithelial  cells  in  the  deep  portion  of  the 
gingival  fold  causes  the  epithelium  to  be  pushed  outward  along  the 
surface  of  the  enamel  and  the  layer  separated  from  the  surface  of 
the  enamel  by  the  action  of  acid  is  this  layer  which  has  been  sepa- 
rated from  the  epithelium  lining  the  gingival  space.  In  this  con- 
nection decalcified  sections  with  all  of  the  tissues  in  relation  should 
be  studied  and  attention  is  called  to  the  comparison  of  the  structure 
of  the  gingival  fold  of  the  tooth  and  the  nail  fold  of  the  finger  nail. 

Nasmyth's  membrane  undoubtedly  has  some  important  relations 
to  normal  and  pathologic  conditions,  especially  those  beginning 
in  the  gingival  space. 

Enamel  Spindles.1 — Especially  in  the  region  of  the  cusps  and  the 
points  where  enamel  formation  begins  in  the  calcification  of  the 
tooth  peculiar  spindle-like  spaces  are  seen  extending  from  the  dento- 
enamel  junction  into  the  enamel.  These  have  often  been  described, 
and  much  has  been  written  in  regard  to  them,  but  there  is  no  agree- 
ment among  investigators  as  to  their  cause  or  significance.  They 
are  apparently  spaces  in  the  interprismatic  substance  and  between 
the  enamel  rods.  They  appear  to  communicate  with  dentinal 
tubules.  In  some  cases  at  least,  they  appear  to  be  filled  with  granu- 
lar material.  They  are  easily  demonstrated,  but  not  so  easily 
explained. 

1  For  further  discussion  of  these  structures  the  student  is  referred  to  Microscopic 
Anatomy  of  the  Teeth  by  Mummery,  p.  78  el  seq. 


CHAPTER  VI. 


THE  DIRECTION  OF  THE  ENAMEL  RODS  IN 
TOOTH  CROWN. 


THE 


IN  describing  the  direction  of  the  enamel  rods  and  their  arrange- 
ment in  what  may  be  called  the  architecture  of  the  tooth  crown, 
they  are  always  considered  as  extending  from  the  dento-enamel 
junction  outward.  This  is  not  only  convenient,  but  logical,  as 
they  are  formed  in  that  way,  beginning  at  the  dento-enamel  junc- 
tion and  being  completed  at  the  surface.  Enamel  is  formed  from 
within  outward,  the  cells  which  produce  it  lying  outside  of  the 
tissue  already  formed,  and  there  are  many  things  about  the  arrange- 
ment of  the  rods  and  their  relation  to  each  other  that  are  understood 
only  when  this  is  borne  in  mind. 

The  direction  of  the  enamel  rods  is  described  by  referring  them 
to  the  horizontal  and  axial  planes,  which  have  been  previously 
defined  (page  35).  The  centigrade  scale,  that  is,  the  division  of 
the  circle  into  one  hundred  equal  arcs,  is  used  because  those  familiar 
with  instrument  nomenclature  are  already  familiar  with  these 
angles,  and  readily  picture  them.1  When  a  rod  is  said  to  be  inclined 
12  centigrades  occlusally  from  the  horizontal  plane,  it  means  that 
if  a  plane  at  right  angles  to  the  long  axis  of  the  tooth  is  passed 
through  the  end  of  the  rod  at  the  dento-enamel  junction,  the  rod 


1  In  the  centigrade  division  the 
circle  is  divided  into  one  hundred 
parts,  each  called  a  centigrade.  One 
centigrade  is  equal  to  3.6  degrees  of 
the  astronomical  circle,  25  centi- 
grades to  90  degrees,  12i  centigrades 
to  45  degrees.  The  cut  gives  a  com- 
parison of  the  two  systems  of  measur- 
ing angles. 


270 


180 
Centigrade  division. 


(77) 


78         DIRECTION  OF  ENAMEL  RODS  IN   TOOTH  CROWN 

will  lie  to  the  occlusal  of  it  and  form  an  angle  of  12  centigrades 
with  it.  In  the  same  way,  if  a  rod  is  said  to  be  inclined  12  centi- 
grades buccally  from  the  mesiodistal  plane,  it  means  that  if  a  plane 
parallel  with  the  axis  of  the  tooth,  and  extending  from  mesio  to 
distal,  is  passed  through  the  end  of  a  rod  at  the  dento-enamel 
junction,  the  rod  will  lie  to  the  buccal  of  it,  and  form  an  angle  of 
12  centigrades  with  it.  By  a  little  practice  with  these  terms  the 
direction  of  the  enamel  rods  can  be  very  easily  and  clearly  pictured 
to  the  mind. 

The  General  Direction  of  Enamel  Rods. — The  general  direction  of 
the  enamel  rods  has  been  variously  described  by  different  authors, 
but  all  of  these  general  statements  are  very  imperfect  and  often 
misleading.  For  instance,  they  are  sometimes  said  to  radiate  from 
the  center  of  the  crown  or  the  pulp  chamber,  but  it  will  be  seen 
that  this  does  not  apply  to  the  rods  which  form  the  lingual  slopes 
of  the  buccal  cusps,  or  the  buccal  slopes  of  the  lingual  cusps  of 
bicuspids  and  molars. 

Again,  they  have  been  said  to  be,  in  general,  perpendicular  to 
the  surface,  but  it  will  be  found  from  the  study  of  sections  that 
there  are  very  few  places  upon  the  surface  where  this  is  true,  and 
that  in  many  places  they  are  far  from  perpendicular  to  the  surface. 
From  a  study  of  sections  it  will  be  seen  that  the  general  arrange- 
ment of  enamel  rods,  in  the  architecture  of  the  tooth  crown  is  such 
as  to  give  the  greatest  strength  to  the  perfect  tissue,  and  to  furnish 
the  greatest  resistance  to  abrasion  in  the  use  of  the  teeth  for  mas- 
tication. In  a  buccolingual  section  through  a  bicuspid  (Fig.  44), 
beginning  at  the  gingival  line,  the  enamel  is  normally  slightly  over- 
lapped by  the  cementum,  and  in  the  gingival  third  the  rods  are 
inclined  more  or  less  apically  from  the  horizontal  plane.  The 
degree  of  inclination  varies  considerably.  It  may  be  as  much 
as  12  centigrades,  but  is  usually  not  more  than  6.  In  general, 
the  more  convex  the  surface,  the  greater  will  be  the  inclination. 
At  some  point  between  the  junction  of  the  gingival  and  middle 
thirds  and  the  middle  of  the  middle  third  of  the  surface  they  are 
in  the  horizontal  plane  and  at  right  angles  to  the  axis  of  the  tooth, 
and  at  this  point  they  are  usually  very  nearly  perpendicular  to 
the  surface.  Passing  occlusally  from  this  point,  they  incline  more 
and  more  occlusally  until  in  the  occlusal  third  they  reach  an  inclina- 
tion of  ]  8  to  20  centigrades  occlusally  from  the  horizontal. 

The  rods  which  form  the  tip  of  the  buccal  cusps  do  not  reach 
the  tip  of  the  dentin  cusp,  but  the  buccal  slope  of  the  dentin.  This 


THE  GENERAL  DIRECTION  OF  ENAMEL  RODS  79 


FIG.  44. — Diagram  of  enamel  rod  directions,  from  a  photograph  of  a  buccolingunl 
section  of  an  upper  bicuspid. 


FIG.  45. — Diagram  of  enamel  rod  directions,  drawn  from  a  mesiodistal  section  of  a 

bicuspid. 


80 


DIRECTION  OF  ENAMEL  RODS  IN  TOOTH  CROWN 


becomes  important,  as  will  be  seen  later.  Over  the  tip  of  the 
dentin  cusp  the  rods  are  in  the  axial  plane,  but  in  this  position  they 
are  usually  very  much  twisted.  Passing  down  the  lingual  slope, 
they  become  more  and  more  inclined  lingual ly  from  the  mesio- 


FIG.  46. — Disturbance  of  enamel  rod  directions  on  labial  surface  of  a  cuspid. 

(About  80  X) 


THE  GENERAL  DIRECTION  OF  ENAMEL  RODS  81 

distal  axial  plane,  and  the  degree  of  inclination  is  related  to  the 
height  of  the  cusp — the  taller  the  cusp,  the  greater  the  inclination. 
At  the  developmental  groove  or  pit  they  meet  the  rods  of  the 
lingual  cups,  which  are  inclined  in  the  opposite  direction. 


FIG.  47  — Disturbance  of  enamel  rod  directions  on  lingual  surface  of  same  tooth  as 
Fig.  48.    (About  80  X) 

6 


82         DIRECTION  OF  ENAMEL  RODS  IN   TOOTH  CROWN 

In  a  mesiodistal  section  (Fig.  45)  the  plan  of  arrangement  will 
be  seen  to  be  the  same,  the  tip  of  the  marginal  ridge  corresponding 
to  the  tip  of  the  cusp.  In  an  incisor  the  arrangement  is  similar, 
the  lingual  marginal  ridge  corresponding  to  a  rudimentary  cusp. 
This  general  plan  should  be  studied  in  several  sections  of  the  various 
classes  of  teeth  before  the  rod  direction  is  studied  more  minutely. 

Effect  of  Hypoplasia. — Whenever  a  hypoplasia  groove  appears 
upon  the  surface,  the  rod  directions  will  be  found  to  be  more  or 
less  disturbed.  Fig.  46  showrs  a  position  on  the  labial  surface  of  a 
cuspid.  In  this  position  the  disturbance  of  the  enamel  rod  direc- 
tion is  very  marked.  The  rods  tend  to  be  in  whorls  and  the  struct- 
ure is  more  or  less  deficient.  On  the  lingual  side  of  the  same  sec- 
tion (Fig.  47)  the  disturbance  in  structure  is  so  great  that  it  is 
difficult  to  make  out  the  rod  direction.  Many  such  areas  will  be 
found  in  sections.  Some  condition  which  has  affected  the  nutri- 
tion of  the  enamel-forming  cells  results  in  a  local  disturbance  of 
the  structural  elements. 

SPECIAL   AREAS. 

The  Gingival  Third. — There  is  much  variation  in  enamel  rod 
direction  in  different  teeth  as  the  gingival  line  is  approached. 
The  inclination  apically  from  the  horizontal  may  be  very  great, 
as  much  as  12  to  15  centigrades  in  some  instances,  as  in  Fig.  48, 
but  this  is  exceptional.  It  may  be  very  slight,  or  the  rods  may 
be  almost  in  the  horizontal  plane.  The  direction  of  the  rods  in 
these  areas  become  very  important  in  the  preparation  of  the  gin- 
gival wall  of  proximal  cavities,  and  cavities  in  the  gingival  third 
of  buccal  and  labial  surfaces. 

The  Tips  of  the  Cusps. — In  studying  the  rod  directions  in  the 
region  of  the  cusps  and  marginal  ridges,  it  must  be  borne  in  mind 
that  the  formation  of  enamel  begins  at  the  dento-enamel  junction, 
at  separate  points,  and  that  the  growth  is  recorded  in  the  tissue 
by  the  bands  of  Retzius,  each  band  having  been  at  one  time  the 
surface  of  the  enamel  cap  then  formed.  In  a  buccolingual  section 
the  formation  of  the  buccal  and  lingual  cusps  will  be  shown 
(Chapter  IX).  While  the  little  caps  are  growing  they  are  being 
carried  apart  by  the  growth  of  the  dental  papilla  and  enamel  organ, 
until  the  calcifications  unite  at  the  dento-enamel  junction.  When 
this  occurs  the  dental  papilla  has  reached  its  maximum  mesiodistal 
diameter.  The  enamel  organ,  however,  will  continue  to  grow,  and 


SPECIAL  AREAS 


83 


as  the  rods  are  completed  first  just  over  the  tip  of  the  dentin  cusp, 
the  continued  growth  causes  an  increase  in  the  inclination  of  the 


FIG    48. — Direction  of  enamel  rods  in  the  gingival  third. 


rods  in  their  outer  portion.     This  often  leads  to  a  curving  of  the 
rods  at  their  outer  portion. 


CHAPTER  VII. 

THE   RELATION   OF   THE   STRUCTURE   TO  THE 
CUTTING  OF  THE  ENAMEL. 

THERE  are  two  methods  of  cutting  enamel — to  chop  or  cleave  it, 
and  to  shave  or  plane  it. 

Cleaving  or  Chopping  Enamel. — In  the  cleavage  of  the  enamel 
the  action  of  the  instrument  more  nearly  resembles  that  of  splitting 
ice  than  that  of  splitting  wood.  The  ax  for  splitting  wood  is  strongly 
wedge-shaped,  and  the  wedge  pries  the  fibers  apart.  In  splitting 
ice  a  small  nick  is  made  on  the  surface  and  then  a  sharp  blow  cracks 
the  ice  in  the  direction  of  the  cleavage.  In  a  similar  way  the  chisel 
applied  to  the  surface  of  the  enamel  makes  a  slight  scratch  or 
bearing  on  the  surface,  and  the  force  applied  at  a  slight  angle  to 
the  direction  of  the  rods  cracks  the  tissue  through  in  the  rod  direc- 
tion. The  bevel  of  the  instrument  is  designed  to  give  strength 
and  keenness  of  edge,  not  to  act  as  a  wedge.  In  order  to  cleave 
the  enamel  it  is  always  necessary  that  there  be  a  break  or  opening 
in  the  tissue,  and  usually  that  the  dentin  be  removed  from  under  it. 
Only  a  small  portion  can  be  split  off  at  a  time.  The  edge  of  the 
chisel  should  be  placed  on  the  enamel  a  quarter  or  half  a  milli- 
meter from  the  opening,  rarely  more,  and  so  piece  after  piece  is 
split  into  the  cavity.  Fig.  49  shows  a  section  of  enamel.  The 
edge  of  the  chisel  is  placed  at  1,  with  the  shaft  in  the  relation  to 
enamel  rod  direction  indicated;  a  tap  of  a  steel  mallet  will  split 
off  a  piece,  and  the  chisel  is  moved  back  to  position  2  and  a  second 
piece  is  split  off.  LTndermined  enamel  will  split  easily  in  this  way. 
As  soon  as  a  point  is  reached  where  the  enamel  rests  on  sound 
dentin,  it  is  recognized  by  the  resistance.  Straight  enamel  can  be 
split  off  from  sound  dentin  without  difficulty  if  attacked  in  the 
proper  way,  but  if  the  inner  portion  is  gnarled  and  twisted,  it  can 
only  be  cleaved  by  removing  the  dentin  from  under  it.  Such  enamel, 
if  resting  on  dentin,  will  split  as  far  as  the  rods  are  straight;  but 
where  they  begin  to  twist  they  will  break  off,  leaving  a  portion 
which  is  very  difficult  to  remove  by  attacking  it  from  the  surface. 
If  the  dentin  is  removed  from  under  gnarled  enamel,  it  will  crack 
(84) 


CLEAVING  OR  CHOPPING  ENAMEL 


85 


through  in  an  irregular  way,  following  the  general  direction  of  the 
rods. 

In  preparing  teeth  for  crowns  it  is  often  necessary  to  remove 
a  large  amount  of  enamel.  This  is  always  more  efficiently  accom- 
plished by  the  intelligent  use  of  sharp  instruments  than  by  force 
alone.  The  enamel  on  axial  surfaces,  especially  in  the  gingival 


FIG.  49. — Position  of  chisel  in  cleaving  enamel. 


half  of  the  crown,  is  usually  straight,  and  if  a  cleavage  line  can 
once  be  established,  the  enamel  can  be  more  easily  and  rapidly 
removed  by  splitting  it  off  piece  after  piece  than  in  any  other  way. 
In  doing  this  a  straight  or  contra-angled  chisel  is  often  the  most 
efficient  instrument,  and  it  must  be  remembered  that  the  "root 
trimmers"  are  more  properly  called  " enamel  cleavers,"  and  that 


86      RELATION  OF  STRUCTURE  TO  CUTTING  OF  ENAMEL 

they  are  used  to  cleave  the  enamel,  not  to  scrape  or  hoe  it  off,  their 
form  being  adapted  to  give  a  strong  palm  grasp  of  the  instrument. 

Fig.  50  illustrates  the  use  of  the  enamel  cleaver  for  the  removal 
of  gingival  enamel  from  an  axial  surface.  The  line  of  cleavage 
being  established,  the  edge  of  the  instrument  is  placed  on  the 


FIG.  50. — The  use  of  enamel  cleaver  in  removing  enamel. 


enamel  half  a  millimeter  from  the  broken  edge,  and  the  force 
which  should  be  strong,  quick,  and  sharp,  is  applied  in  the  direc- 
tion indicated,  and  piece  after  piece  is  split  off,  progressing  from 
the  occlusal  toward  the  gingival.  In  preparing  the  wall  of  a  cavity 
the  outline  form  should  be  attained  by  cleavage,  and  this  is  the 
first  step  in  the  preparation  of  the  cavity. 

After  the  enamel  has  been  removed  by  cleavage  to  the  point 


SHARP  INSTRUMENTS 


87 


where  the  margin  is  to  be  laid,  the  wall  must  be  completed  by 
cutting  the  enamel  in  an  entirely  different  way. 

Planing  or  Shaving  Enamel. — In  this  manner  of  cutting  enamel 
the  tissue  is  removed  without  reference  to  the  rod  direction,  and 
without  injury  to  its  structure  (Figs.  51,  52,  and  53).  The  chisel 
is  used  like  the  blade  of  a  plane.  The  cutting  edge  is  placed  against 
the  surface  with  the  shaft  of  the  instrument  almost  parallel  to  it, 
and  the  tissue  is  shaved  away.  In  this  way  the  rods  that  have 
been  cracked  apart  by  the  cleavage  are  removed,  and  the  walls 
arranged  in  terms  of  its  structural  elements  so  as  to  gain  the 
required  strength  of  margin. 

Sharp  Instruments. — Chisels  and  hatchets  for  use  in  cleaving  or 
planing  enamel  must  be  keenly  sharp.  If  a  dull  edge  is  placed  on 


FIG.  51 


FIG.  52 


FIG.  53 


FIGS.  51,  52,  and  53. — The  use  of  the  chisel  in  planing  or  shaving  enamel.     (Black.) 

the  surface  of  the  enamel  it  will  rest  across  the  ends  of  many  rods, 
and  force  applied  will  only  crumble  them,  but  will  not  split  the 
tissue.  The  edge  must  be  keen  (Fig.  54),  so  as  to  engage  between 
the  rods  and  so  start  the  cleavage.  Cutting  instruments  as  fur- 
nished by  dental  supply  houses  are  not  tempered  hard  enough  to 
hold  an  edge.  There  is  no  fault  to  be  found  with  the  supply  houses 
for  this,  for  they  make  them  as  the  dentist  wants  them,  and  any 
dealer  will  furnish  hard-tempered  instruments  if  they  are  ordered. 
To  use  hand  instruments  successfully  in  cutting  enamel,  the  stock 
instruments  must  either  be  retempered  or  they  must  be  ordered 
hard  tempered.  The  cutting  edge  of  the  blade  of  an  enamel  instru- 
ment should  be  straw-colored  when  tempered. 

The  chisel  and  hatchets  are  the  instruments  for  removing  enamel. 


88       RELATION  OF  STRUCTURE  TO  CUTTING  OF  ENAMEL 

The  burr  is  the  instrument  for  removing  hard  dentin.  When  the 
burr  is  used  on  enamel  it  should  be  remembered  that  it  is  used  as 
a  revolving  chisel.  It  is  by  the  thoughtful  use  of  hand  instruments 


FIG.  54. — The  relation  of  the  edge  of  a  sharp  and  a  dull  chisel. 
FIG.  55  FIG.  56  FIG.  57 


FIGS.  55,  56,  and  57. — The  use  of  the  chisel  in  cleaving  enamel.    Opening  an  occlusal 

cavity.     (Black.) 


SHARP  INSTRUMENTS  89 

that  knowledge  of  enamel  rod  direction  is  gained,  and  only  by 
the  use  of  them  can  the  enamel  walls  be  prepared  in  terms  of  their 
structural  elements.  In  cleaving  undermined  enamel  the  edge 
may  be  used  either  with  a  pulling  or  a  pushing  motion.  For 
instance,  in  opening  a  cavity  in  the  occlusal  surface  of  a  bicuspid, 
the  buccal  portion  of  undermined  enamel  is  split  off  by  placing 
the  instrument  as  shown  in  Figs.  55  and  56.  The  bevel  of  the 
blade  is  held  toward  the  cavity  and  the  shaft  of  the  instrument  at 
a  slight  angle  to  the  rod  direction,  and  the  force  is  applied  in  the 
direction  of  the  shaft.  The  lingual  portion  may  be  removed  by 
placing  the  instrument  as  indicated  in  Fig.  57,  the  bevel  of  the 
blade  away  from  the  cavity  and  the  force  applied  in  the  direction 
of  the  bevel  by  a  pulling  force  in  the  direction  of  the  shaft.  This 
is  the  way  in  which  force  is  applied  on  enamel  cleavers.  The  pitch 
of  the  bevel  in  an  enamel  cleaver  and  its  relation  to  the  shaft  of 
the  instrument  is  extremely  important,  and  the  efficiency  of  an 
instrument  may  easily  be  ruined  by  careless  honing.  Every  time 
a  cutting  instrument  is  applied  to  the  enamel  it  must  be  done  with 
a  knowledge  of  the  relation  of  the  cutting  edge  and  the  force  to 
the  direction  of  the  enamel  rods,  until  it  becomes  entirely  auto- 
matic. The  author  emphatically  believes  that  the  acquirement  of 
this  knowledge  and  skill  will  do  more  to  increase  facility  and  suc- 
cess in  the  preparation  of  cavity  walls  than  any  other  manipulative 
factor.  The  preparation  of  enamel  walls  requires  the  continual 
application  of  the  knowledge  of  enamel  structure.  Enamel  is  a 
very  hard  tissue,  but  it  is  composed  of  structural  elements,  and 
walls  prepared  without  reference  to  them  will  prove  their  own 
weakness. 


CHAPTER  VIII. 

THE  STRUCTURAL  REQUIREMENTS   FOR  STRONG 
ENAMEL  WALLS. 

FROM  the  consideration  of  the  physical  character  of  the  enamel, 
its  structural  elements  and  their  properties,  it  is  evident  that  the 
strength  of  any  enamel  wall  is  dependent  upon  the  arrangement 
of  the  rods  in  the  tissue  which  makes  up  the  walls  and  their  relation 
to  the  dentin.  Certain  requirements  for  strength  can  be  clearly 
stated,  and  these  are  applicable  to  all  enamel  walls.  They  cannot 
always  be  secured  with  equal  facility  or  perfection,  but  in  propor- 
tion as  these  principles  are  observed  and  attained  the  wall  will  be 
strong;  as  they  are  imperfectly  attained  or  ignored  the  wall  will 
be  weak  and  unreliable.  When  these  conditions  are  understood 
very  many  failures  can  be  clearly  seen  to  have  been  the  result  of 
their  neglect. 

Structural  Requirements. — 1.  The  enamel  must  rest  upon  sound 
dentin. 

2.  The  rods  which  form  the  cavosurface  angle  must  have  their 
inner  ends  resting  upon  sound  dentin. 

3.  The  rods  which  form  the  cavosurface  angle  must  be  supported 
by  a  portion  of  enamel  in  which  the  inner  ends  of  the  rods  rest  on 
sound  dentin  and  the  outer  ends  are  covered  by  the  filling  material. 

4.  The  cavosurface  angle1  must  be  trimmed  or  bevelled  so  that 
the  margin  will  not  be  liable  to  injury  in  condensing  the  filling 
material  against  it  (Fig.  58). 

These  requirements  should  be  considered  one  by  one. 

The  Enamel  Must  Rest  upon  Sound  Dentin. — That  is,  the  enamel 
plate  must  have  the  support  of  sound  dentin,  and  all  portions  which 
are  undermined  by  the  removal  of  dentin  must  be  cut  away.  When 
the  inner  ends  of  the  rods  which  form  the  enamel  plate  rest  upon 
sound  dentin,  the  elasticity  of  the  dentin  gives  to  the  enamel  a 
certain  degree  of  elasticity,  but  the  enamel  itself  without  this  support 

1  The  cavosurface  angle  is  defined  as  the  angle  formed  by  the  surface  of  the  tooth 
and  the  wall  of  the  cavity. 

(90) 


ENAMEL  MUST  REST   UPON  SOUND  DEN  TIN 


91 


is  extremely  brittle.  A  force  that  causes  it  to  give  will  crack  it 
through  its  entire  thickness.  No  filling  material  or  substitute  for 
the  lost  dentin  can  restore  the  original  conditions.  Figs.  58  and  59 
illustrate  these  requirements.  The  enamel  plate  a,  b,  c,  d  rests  upon 
sound  dentin.  The  rods  which  form  the  cavosurface  angle  at  b 
run  uninterruptedly  to  the  dentin,  and  their  inner  ends  rest  on  it 


»  PP0 

- 


FIG.  58.  —  The  structural  requirements  for  a  strong  enamel  wall. 


at  e.  The  rods,  b,  e  are  also  supported  by  a  portion  of  enamel, 
a,  b,  e,  made  up  of  rods  whose  inner  ends  rest  upon  the  dentin  and 
whose  outer  ends  are  covered  in  by  the  filling  material,  altogether 
supporting  the  marginal  rods  like  a  buttress.  And  the  cavosurface 
angle  is  bevelled,  including  from  |  to  -1-  of  the  enamel  wall,  so  as  to 
remove  the  sharp  corner  which  would  be  in  danger  of  crumbling 
under  an  instrument.  An  enamel  wall  should  be  considered  no 


92      STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

stronger  after  the  filling  is  inserted  than  it  was  before.  Moreover, 
when  the  dentin  has  been  decalcified  or  destroyed  by  the  action 
of  caries,  the  acid  which  has  decalcified  the  dentin  has  also  acted 
upon  the  enamel,  dissolving  the  cementing  substance  from  between 
the  rods,  from  within  outward,  often  to  a  great  extent,  and  the 
structure  is  very  imperfect.  Enamel  that  has  been  so  weakened 


vJ^^^^il^^-^'vV'afKv^-^/'^^jS, 


FIG.  59. — -The  structural  requirements  for  a  strong  enamel  wall:  a,  b,  the  level 
of  the  cavosurface  angle.  The  rods  forming  the  margin  of  the  cavity  at  b  reach  the 
dentin  at  e,  and  are  supported  by  the  portion  a,  b,  e. 


will  not  withstand  the  force  of  mastication,  and  sooner  or  later  will 
crack  or  break  away  from  the  filling  material.  It  should  be  removed 
and  the  wall  formed  in  tissue  whose  structure  is  perfect.  Occasion- 
ally cases  arise  where  an  operator  decides  to  leave  some  unsupported 
enamel,  but  its  weakness  and  the  possibility  of  restoring  it  if  it 


ENAMEL  MUST  REST   UPON  SOUND  DENTIN 


93 


breaks  away  without  destroying  the  original  operation  must  always 
be  considered.  It  is  sometimes  supposed  that  it  is  only  necessary 
to  have  sound  enamel  resting  on  sound  dentin,  but  by  looking  at 
Figs.  60  and  61  it  will  be  seen  that  the  first  requirement  may  be 
present,  but  not  the  second.  In  these  illustrations  the  enamel 
plate  is  resting  on  sound  dentin,  but  the  tissue  has  been  cut  in  such 


i  mitmmk 


FIG    60. — Improperly  prepared  enamel  wall.     The  portion  a,  b.  c  has  the  inner  ends 
of  the  rods  cut  off  and  they  do  not  reach  the  dentin. 

a  way  that  the  inner  ends  of  the  rods  have  been  cut  off.  The  rods 
that  form  the  cavosurface  angle  do  not  extend  to  the  dentin,  but 
run  out  on  the  cavity  wall  at  d,  and  the  portion  a,  b,  c  is  held  to- 
gether only  by  the  cementing  substance.  This  is  not  strong  enough 
to  sustain  the  force  necessary  to  condense  the  filling  material  or 
the  forces  received  upon  the  surface  of  the  tooth  after  the  filling 
is  completed.  It  will  crack  on  the  line  of  the  cementing  substance 


94         STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

and  chip  out.  The  inclination  of  the  entire  wall  must  be  increased 
to  a  little  more  than  to  reach  the  rod  direction.  Such  a  wall  as 
this  may  easily  be  made,  in  preparing  a  cavity  wall,  with  a  stone 
or  a  burr,  but  would  not  be  liable  to  be  formed  with  hand  instru- 
ments. Such  walls  as  this  account  for  the  chipping  of  many  margins 


Mt/ KfL'-i'.trv.  hTviyi  *-• 


FIG.  61. — Improperly  prepared  enamel  wall.     The  portion  a,  b,  c  is  not  supported 

by  dentin. 


and  the  failure  of  fillings  along  the  gingival  wall.  The  tissue  is 
cracked  to  pieces  in  inserting  the  filling  material,  and  the  pieces 
fall  out  later.  This  occurs  often  in  the  gingival  walls  of  compound 
cavities. 

The  Rods  Forming  the  Cavosurface  Angle  Must  be  Supported. — 
This  is  the  key  to  strong  enamel  walls.  The  more  perfect  the  sup- 
port the  stronger  the  wall.  If  an  enamel  wall  is  cut  exactly  in  the 
direction  of  the  rods,  as  in  Fig.  62,  the  rods  forming  the  margin 


THE  RODS  FORMING   THE  CAVOSURFACE  ANGLE 


95 


are  held  together  only  by  cementing  substance,  and  a  compara- 
tively slight  force  on  the  surface  in  the  direction  toward  the  cavity 
will  break  them  off.  If  the  same  wall  is  trimmed,  as  indicated  by 
the  line,  the  same  force  would  do  no  damage,  as  the  rods  which 
receive  it  are  supported  by  the  portion  which  is  covered  by  the  filling 


FIG.  62. — Enamel  wall  cut  in  the  direction  of   the  rods.      The  marginal  rods  are  not 
supported.    It  should  be  trimmed  in  the  line  indicated. 


material.  It  is  interesting  to  note  that  in  the  wearing  down  of  the 
enamel  by  use,  nature  provides  the  same  support  for  the  rods 
which  form  the  angle  of  the  worn  and  tooth  surfaces.  Fig.  63 
shows  the  tip  of  a  worn  incisor.  The  rods  at  A  reach  the  dentin 
at  C  and  are  supported  by  the  portion  A,  B,  C.  When  caries  occurs 
on  an  abraded  surface  it  starts  by  the  rods  at  the  dento-enamel 


96         STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

junction,  chipping  out  and  forming  a  protected  niche  for  the  lodg- 
ment of  a  colony. 

Bevel  the  Cavosurface  Angle. — It  is  not  always  necessary  to  bevel 
the  cavosurface  angle  where  the  rods  are  inclined  toward  the 
cavity.  In  such  places  the  rods  forming  the  margin  are  well  sup- 
ported and  the  angle  need  not  be  bevelled  unless  it  is  so  sharp  that 
it  would  be  in  danger  of  being  injured. 

There  are  two  reasons  for  bevelling  the  cavosurface  angle:  (1) 
To  protect  a  sharp  angle  from  injury;  (2)  to  gain  support  for  the 


FIG.  63. — The  tip  of  a  worn  incisor.     The  rods  forming  the  angle  at  A  reach  the 
dentin  at  C,  and  are  supported  by  the  piece  A,  B,  C. 


marginal  rods.  The  first  occurs  where  the  enamel  rods  are  inclined 
toward  the  cavity,  the  second  where  they  are  inclined  away  from 
the  cavity. 

Classes  of  Cavities. — From  a  consideration  of  the  direction  of 
the  enamel  rods  in  the  tooth  crown,  and  the  positions  where  caries 
begins  on  the  enamel,  enamel  walls  may  be  divided,  according  to 
their  structural  type,  into  two  classes  (Fig.  64): 

1.  Those  in  which  the  enamel   rods   are   inclined   toward   the 
cavity,  characteristic  of  cavities  on  occlusal  surfaces  and  cavities 
beginning  in  fissures  and  pits. 

2.  Those  in  which  the  enamel  rods  are  inclined  away  from  the 
cavity,  characteristic  of  cavities  on  smooth  surfaces. 


CLASSES  OF  CAVITIES 


97 


In  the  first  class  it  is  comparatively  easy  to  obtain  a  strong 
margin,  and  this  is  fortunate,  for  when  the  filling  is  completed  the 
margin  will  be  subjected  to  the  full  force  of  mastication.  In  the 
second  it  is  comparatively  difficult  to  obtain  a  strong  margin, 
but  only  sufficient  strength  is  required  to  withstand  the  force  of 
condensing  the  filling  material,  as  after  the  filling  is  completed  it 
will  be  obliged  to  withstand  little  force  from  mastication. 

From  a  careful  observation  of  the  failures  of  fillings  (his  own 
and  those  of  other  operators),  the  author  believes  a  very  large 


FIG.  64. — The  two  classes  of  cavities.    Those  with  the  rods  inclined  toward  the  cavity, 
and  those  with  the  rods  inclined  away  from  the  cavity. 


number  are  due  to  structurally  imperfect  enamel  walls.  A  study 
of  enamel  structure  as  related  to  cavity  preparation  will  do  more 
to  improve  the  quality  of  the  operation  and  to  increase  the  facility 
of  its  execution  than  any  one  factor.  This  study  is  a  clinical  study 
guided  by  examination  of  the  microscopic  structure  of  the  tissue. 
In  operating  at  the  chair  the  detail  of  enamel  rod  direction  as  it  is 
applied  to  cavity  preparation  is  learned,  but  to  do  so  hand  instru- 
ments must  be  used  and  a  sufficient  knowledge  of  the  tissue  must 
have  been  acquired  to  think  of  it  always  in  their  use  in  terms  of 
its  structural  elements. 


98         STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

The  steps  in  the  preparation  of  an  enamel  wall  are: 

1.  The  cleavage  of  the  enamel  until  the  outline  form  of  the 
cavity  is  reached. 

2.  The  trimming  of  the  enamel  walls. 

3.  The  preparation  of  the  margins. 


FIG.  65. — Occlusal    fissure  in   an   upper   bicuspid,   showing   direction   of  rods. 

(About  80  X) 

Every  enamel  wall  should  be  prepared  according  to  these  steps. 
The  first  not  only  removes  the  tissue  more  or  less  disintegrated 
and  weakened  by  caries,  but  also  places  the  margin  of  the  filling 
in  a  position  where  it  is  not  likely  to  be  covered  by  the  growth  of 


CLASSES  OF  CAVITIES 


99 


a  colony  of  bacteria.  It  also  determines  the  direction  of  the  enamel 
rods  so  that  the  walls  can  be  completed  in  terms  of  its  structural 
elements. 


FIG.  66. — The  same  section  as  Fig.  65,  showing  the  position  of  the  chisel  in  cleaving 
the  enamel  to  open  the  cavity. 


The  second  step  is  accomplished  by  the  shaving  or  planing  pro- 
cess, and  should  always  increase  the  inclination  of  the  entire  enamel 
wall  slightly,  so  as  to  extend  a  little  beyond  the  rod  directions, 
and  remove  the  portions  that  have  been  cracked  or  splintered  by 


100     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

the  cleavage.  After  cleavage  the  enamel  wall  will  usually  have 
a  more  or  less  whitish  look.  This  is  caused  by  the  cracking  of  the 
cementing  substance  between  the  rods.  The  light  is  refracted  by 


FIG.  67. — Preparation  of  enamel  walls  in  occlusal  fissure  cavities  (the  same  as  Figs. 

65  and  66). 


the  air  in  these  microscopic  spaces  and  imparts  this  whitish  or 
snowy  look  to  the  tissue.  These  portions  are  removed  by  planing 
or  shaving,  and  the  tissue  obtains  its  bluish,  translucent  appearance. 


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CLASSES  OF  CAVITIES 


101 


The  third  step  is  also  accomplished  by  the  planing  process,  and 
should  be  carried  out  with  two  objects  in  mind:  (1)  To  so  form 
the  cavosurface  angle  that  the  tissue  will  not  be  liable  to  injury 
in  the  condensation  of  the  filling  material  against  it,  and  (2)  to 
leave  rods  whose  outer  ends  will  be  covered  by  the  filling  material 
to  support  those  which  form  the  actual  margin  of  the  cavity. 

The  steps  in  the  preparation  of  enamel  walls  may  be  made  more 
clear  by  photomicrographs.  Plate  VII  shows  a  portion  of  enamel 
close  to  a  carious  cavity  which  is  to  be  extended  to  the  left.  The 
chisel  is  placed  close  to  the  margin  and  the  portion  is  split  off.  The 


FIG.  68. — The  relation  of  the  cavity  to  the  crown  (the  same  as  Figs.  66  and  67). 

wall  then  appears  whitish,  for,  as  is  seen,  the  cementing  substance 
has  cracked  in  several  places,  disturbing  the  structure,  and  in 
several  places  rods  have  been  broken  across.  The  wall  must  now 
be  planed  so  as  to  increase  the  inclination  of  the  entire  wall  slightly, 
and  finally  the  cavosurface  angle  must  be  bevelled,  involving  from 
\  to  \  of  the  thickness  of  the  enamel  wall  to  give  support  to  the 
rods  forming  the  margins.  In  this  case  the  rods  are  straight  and 
parallel,  but  in  Plate  VIII  they  are  twisted.  If  the  dentin  is  removed 
from  under  this  enamel  and  the  chisel  placed  as  indicated,  the  por- 
tion will  be  split  out,  but  not  only  has  the  tissue  been  splintered, 


102     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

but  a  considerable  portion  is  left  in  which  the  rods  have  been 
broken  across.  By  feeling  of  the  margin  with  the  chisel  this  can 
easily  be  determined,  and  the  angle  of  the  wall  must  be  increased 
by  planing  so  as  to  leave  the  wall  in  the  position  shown  in  Plate 
VIII,  3,  and  finally  the  cavosurface  angle  must  be  bevelled. 


FIG.   09. — -The  trimming  of  the  walls  instead  of  lapping  the  filling  material  on  the 

slope  of  the  cusps. 

Preparation  of  Simple  Occlusal  Cavities. — Caries  often  begins 
in  the  mesial  and  distal  pits  of  the  upper  bicuspids,  and  in  pre- 
paring the  cavities  for  filling  they  must  be  united.  Fig.  65  is  a 
buccolingual  section  through  a  first  superior  bicuspid.  Suppose 
caries  has  reached  the  dento-enamel  junction  in  both  the  mesial 
and  distal  pits,  and  they  are  to  be  united  along  the  groove.  A 
small  spear  drill  is  carried  into  the  mesial  pit  until  the  dento-enamel 
junction  is  reached,  then  a  small  inverted  cone  burr  is  carried  into 
the  dentin  just  under  the  enamel  and  drawn  from  the  dentin  to 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES 


103 


the  surface  of  the  enamel.  When  a  narrow  cut  has  been  made 
from  the  mesial  to  the  distal  pit,  a  chisel  placed  at  the  edge  of  the 
opening  will  split  out  the  enamel  as  indicated  in  Fig.  71.  Now  the 
walls  must  be  planed  so  as  to  bring  the  buccal  and  lingual  walls 
into  the  axial  plane,  and  the  structural  requirements  will  have 


FIG.  70.— Caries  beginning  in  an  occlusal  defect  of  a  molar.     (About  80  X) 

been  completed  (Fig.  67).    Fig.  68  shows  the  relation  of  the  cavity 
to  the  crown. 

It  has  often  been  advised  to  allow  the  filling  to  extend  on  to 
the  natural  slopes  of  the  cusps,  as  indicated  in  Fig.  69.  It  will 
be  seen,  however,  that  a  stronger  enamel  wall  and  a  stronger  edge 


104      STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

of  filling  material  will  be  obtained  if  the  enamel  wall  is  bevelled 
to  the  point  where  the  margin  of  the  filling  is  desired  and  the  filling 
finished  to  this  position. 

Fig.  70  shows  a  buccolingual  section  through  a  molar  with  a 
small  cavity  in  a  mesial  pit.     Caries  has  undermined  the  enamel 


FIG.  71. — The  preparation  of  the  enamel  walls  of  the  cavity  shown  in  Fig.  70. 

slightly  toward  the  buccal,  but  has  attacked  the  enamel  on  the 
surface,  extending  toward  the  lingual  farther  than  the  enamel 
has  been  undermined  at  the  dento-enamel  junction.  Applying 
the  chisel  to  the  surface,  the  undermined  enamel  is  split  away, 
as  is  indicated  in  Fig.  71.  The  buccal  wall  is  planed  until  it  is  in 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES        105 


the  axial  plane,  and  the  cavosurface  angle  bevelled.  It  is  not 
necessary  to  extend  the  cavity  to  the  lingual  beyond  the  point 
where  sound  dentin  is  reached,  but  the  disintegrated  enamel  on 


Fio.  72. — The  relation  of  the  cavity  to  the  crown  (the  same  section  was  shown  in 

Figs.  70  and  71). 


H  j'^illiiliihlHiiiii'l'M 

FIG.    73. — A  larger  cavity  in  the  occlusal  surface  of  a  molar.     The  position  of  the 
chisel  in  opening  the  cavity. 


106     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

the  surface  must  be  removed.  The  enamel  wall  is  therefore 
inclined  about  6  centigrades  lingually  from  the  axial  plane,  and  it 
is  not  necessary  to  bevel  the  cavosurface  angle.  The  rods  are 
inclined  toward  the  cavity,  the  rods  forming  the  margins  are 


FIG.  74. — A  gingival  third  cavity  in  a  bicuspid,  showing  the  cleavage  of  the  occlusal 
and  gingival  walls  as  cleaved. 


PREPARATION  OF  SIMPLE  OCCLUSAL  CAVITIES        107 

well  supported,  and  the  cavosurface  angle  is  not  so  sharp  as  to  be 
endangered  in  condensing  filling  material.  Fig.  72  shows  the  rela- 
tion of  the  cavity  to  the  crown. 


FIG.  75. — The  preparation  of  the  cavity  shown  in  Fig.  74. 

All  occlusal  defects  should  be  filled  as  soon  as  the  decay  has 
reached  the  dento-enamel  junction,  as  all  progress  of  the  disease 


108     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

beyond  that  point  requires  sacrifice  of  tissue  which  otherwise 
would  be  saved,  and  the  enamel  wall  becomes  less  and  less  strong. 
Fig.  73  shows  a  much  more  extensive  occlusal  cavity,  one  that 
has  been  neglected  until  the  enamel  has  been  broken  in,  and  as  a 
result  there  was  much  unnecessary  loss  of  tooth  structure.  The 
chisel  is  applied  to  the  surface  as  indicated,  and  the  undermined 
enamel  broken  down  until  the  sound  dentin  is  reached.  On  the 
buccal,  the  enamel  wall  is  cut  to  the  axial  plane,  and  the  cavosur- 
face  angle  bevelled.  If  the  decay  in  the  dentin  had  reached  the 
tip  of  the  dentin  cusp,  it  would  be  necessary  to  remove  the  tip  of 
the  enamel  cusp  and  incline  the  wall  about  8  centigrades  buccally 


FIG.  76. — A  gingival  third  cavity  in  a  molar. 

from  the  axial  plane,  in  order  to  obtain  a  strong  wall,  and  then 
the  cusp  would  be  replaced  by  filling  material.  On  the  lingual 
the  undermined  enamel  is  removed,  and  the  wall  inclined  slightly 
lingually  from  the  axia  1  plane  and  the  cavosurface  angle  bevelled 
a  little.  Fig.  73  shows  the  relation  of  the  cavity  to  the  crown. 

Gingival  Third  Cavities. — Fig.  74  is  a  buccolingual  section  of  a 
superior  bicuspid,  showing  a  break  in  the  enamel  in  the  position 
of  a  gingival  third  cavity.  The  occlusal  wall  is  cleaved  to  find  the 
enamel  rod  direction,  then  planed  to  increase  the  inclination  slightly, 
leaving  it  inclined  about  8  centigrades  occlusally  from  the  hori- 
zontal plane,  and  the  cavosurface  angle  bevelled  to  obtain  support 


GINGIVAL  THIRD  CAVITIES 

FIG.  77 


109 


1.   Wall  as  cleaved. 


FIG.  78 


2.   Wall  as  trimmed. 
FIGS,  77  and  78.— Preparation  of  occlusal  wall  of  Fig.  76.    (About  70  X). 


110     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL  WALLS 

for  the  marginal  rods.  The  gingival  wall  is  prepared  in  the  same 
way,  inclined  gingivally  about  6  centigrades  from  the  horizontal 
plane,  and  the  cavosurface  angle  bevelled.  Fig.  75  shows  the  walls 
prepared. 


FIG.  79. — A  cavity  in  the  lingual  pit  of  a  lateral  incisor.     The  position  of  the  chisel 

in  opening  the  cavity. 

Fig.  76  is  a  similar  section  from  a  molar.  After  chopping  away 
the  occlusal  wall  until  the  cavity  has  been  extended  to  the  point 
of  greatest  convexity  of  the  surface,  the  wall  is  seen  to  be  in  the 


FIG.  80. — The  preparation  of  the  gingival  wall  of  the  cavity  shown  in  Fig.  79. 


\ 


FTG.  81. — The  preparation  of  the  cavity  shown  in  Fig.  79, 


112     STRUCTURAL  REQUIREMENTS  FOR  ENAMEL   WALLS 

condition  shown  in  Fig.  77.  Near  the  surface  the  enamel  has  broken 
across  the  rods  and  near  the  dento-enamel  junction  the  same  thing  has 
happened,  but  in  the  rest  of  the  distance  the  cleavage  has  followed 
the  enamel  rod  direction.  The  inclination  of  the  wall  is  increased 
by  planing  until  this  roughness  has  been  removed,  and  then  the 
cavosurface  angle  is  bevelled  to  support  the  marginal  rods,  and 
preparation  is  complete,  as  shown  in  Fig.  78. 

Fig.  79  shows  a  cavity  in  the  lingual  pit  of  a  superior  lateral 
incisor.  Caries  has  undermined  the  enamel  to  a  considerable 
extent,  and  the  cavity  will  have  to  be  larger  than  would  otherwise 
have  been  necessary.  Placing  the  chisel  close  to  the  occlusal 
margin,  as  indicated,  the  enamel  is  chipped  away  in  that  direction 
and  around  the  circumference.  On  the  lingual  wall  the  chisel 
may  be  reversed  and  used  with  a  pulling  motion,  like  a  hoe.  In 
this  way  the  undermined  enamel  is  chipped  away  and  the  tip  of 
the  marginal  ridge  removed.  The  wall  is  then  planed  into  the 
horizontal  plane  and  the  cavosurface  angle  bevelled.  Fig.  80 
shows  the  structure  of  the  gingival  wall,  and  Fig.  81  the  relation 
to  the  crown. 


CHAPTER  IX. 

STRUCTURAL  DEFECTS  IN  THE  ENAMEL. 

THE  formation  of  enamel  begins  at  the  dento-enamel  junction, 
and  the  tissue  is  laid  down  from  within  outward,  so  that  the  enamel 
in  contact  with  the  dentin  is  formed  first  and  the  surface  of  the 
crown  last.  Enamel  formation  begins  at  several  points  for  each 
crown,  the  exact  number  and  position  of  which  has  been  the  sub- 
ject of  much  investigation.  When  enamel  formation  begins, 
these  points  are  close  together,  but  they  are  carried  farther  apart 
by  the  growth  of  the  dental  papilla,  and  are  not  united  for  some 
time.  The  separate  enamel  caplets  unite  first  at  the  dento-enamel 
junction,  and  as  the  formation  of  the  thickness  of  the  enamel 
progresses  at  these  lines  of  union,  there  is  always  more  or  less 
disturbance  in  structure.  Even  \vhere  the  union  seems  perfect, 
sections  will  show  more  or  less  disturbance  of  enamel-rod  direction, 
arrangement  of  the  rods,  and  relation  to  the  cementing  substance. 

Every  operator  and  student  of  dental  anatomy  is  familiar  with 
the  developmental  lines.  On  the  occlusal  surfaces  they  are  usually 
marked  by  well-defined  grooves,  but  upon  the  axial  surfaces  the 
grooves  may  be  very  slight,  scarcely  more  than  slight  depressions 
of  the  surface,  and  consequently  the}'  are  not  thought  of.  It  will 
be  found,  however,  that  on  these  lines  there  is  less  perfect  enamel 
structure,  and  consequently  the  tissue  is  not  as  strong,  and  these 
lines  must  be  avoided  in  the  preparation  of  enamel  walls.  The 
cause  of  disturbance  of  structure  will  be  better  understood  after 
study  of  the  development  of  the  tooth  germ  and  the  formation 
of  enamel  in  the  chapter  on  Dental  Embryology,  but  some  details 
of  the  cause  should  be  touched  upon  here.  The  study  of  the  dia- 
grams of  the  growth  of  the  tooth  crown  will  illustrate  the  conditions 
(see  Chapter  XXVI,  Fig.  278),  and  shows  a  buccolingual  section 
through  the  tooth  germ  of  a  bicuspid  just  before  the  formation 
of  the  dentin  and  the  enamel  begins.  The  odontoblasts  (dentin- 
forming  cells)  and  the  ameloblasts  (enamel-forming  cells)  are  in 
contact  at  what  will  be  the  dento-enamel  junction.  The  odonto- 
blasts form  dentin  on  their  outer  surface,  beginning  at  the  tip  of 
8  (113) 


114 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


the  dentin  cusp,  and  progress  from  without  inward  and  extend 
down  the  slopes  of  the  cusps.  The  ameloblasts  form  enamel  on 
their  inner  surface  and  progress  from  within  outward  and  down 


FIG.  82 


FIG.  83 


FIG.    84 


FIG.  85 


FIG.  86 


FIG.  87 


FIGS.  82  to  87. — Diagrams  showing  the  growth  of  the  crown  of  a  bicuspid. 

the  slopes  of  the  cusps.  In  this  way  little  caplets  of  dentin  covered 
by  enamel  are  formed  over  the  horns  of  the  dental  papilla;  the 
caps  are,  of  course,  thickest  where  formation  has  been  going  on 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


115 


longest.    While  these  caps  are  forming,  the  dental  papilla  is  increas- 
ing in  size,  and  so  they  are  carried  farther  and  farther  apart  (Figs. 


FIG.  88. — The  section  from  which  Figs.  82  to  87  were  drawn:  A,  tip  of  den  tin  cusp; 
B,  lines  showing  little  caps  of  enamel  formed  before  calcifications  from  separate 
centres  united;  C,  lines  showing  amount  of  enamel  formed  when  calcifications 
united. 


FIG.  89.- — Occlusal  defects  from  an  old  tooth. 

82  to  87).     As  soon  as  the  calcifications  reach  each  other  at  the 
dento-enamel  junction  and  unite,  the  increase  in  the  diameter  of 


116  STRUCTURAL  DEFECTS  IN   THE  ENAMEL 

the  dental  papilla  ceases.     The  layer  of  ameloblasts,  which  are 
tall  columnar  cells,  now  cover  the  surface  of  the  enamel  and  receive 


FIG.  90. — A  deep,  open  groove 


FIG.  91. — A  shallow  groove. 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


111 


FIG.  92. — A  very  deep  groove,  showing  the  effect  of  caries  at  the  bottom. 


FIG.  93. — The  pit  in  a  lateral  incisor  filled  with  coronal  cementum.      Interglobular 
spaces  are  seen  in  the  den  tin. 


118 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


their  nourishment  and  the  materials  for  the  formation  of  enamel 
from  the  blood  supply  through  the  stratum  intermedium.  As 
the  blood  supply  comes  from  above,  it  is  evident  that  the  cells 
high  up  along  the  slopes  of  the  cusps  will  receive  most,  while  those 


FIG  94 


FIG.  95 


FIG.  94. — Occlusal  surface  of  the  lower  third  molar,  showing  the  grooves. 
FIG.  95. — The  same  tooth  sliced  for  sectioning:    1,  the  piece  from  which  the  section 
shown  in  Figs.  96  and  97  was  ground. 

at  the  bottom  of  the  groove  get  what  is  left.  The  formation  is 
therefore  more  rapid  along  the  slopes  and  less  rapid  at  the  point 
of  union.  As  growth  continues,  this  difference  in  supply  increases, 
and  accordingly  formation  at  the  bottom  of  the  groove  is  first 


FIG.  9fi. — The  section  ground  from  1,  Fig.  95,  showing  the  depth  of  the  fissure. 

slowed  and  finally  stopped,  and  the  result  is  a  defect.    The  taller 
the  cusps  the  greater  will  be  the  interference  and  the  deeper  the 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


119 


defective  groove.  In  studying  sections  (Figs.  88  to  92)  it  is  very 
noticeable  that  teeth  with  long  pointed  cusps  have  more  open 
grooves,  and  the  defect  often  extends  almost  or  quite  to  the  dento- 
enamel  junction. 


FIG.  97. — Higher  magnification  of  the  fissure  shown  in  Fig.  96.     (About  GO  X) 

The  bands  of  Retzius,  which  are  the  incremental  lines  of  the 
enamel  about  these  grooves,  should  be  studied.  It  will  be  seen 
that  they  always  dip  down  around  the  groove,  and  that  more 
enamel  has  been  formed  between  one  band  (Figs.  98  and  99)  and 
the  next  on  the  slope  of  the  cusps  than  at  the  bottom  of  the  groove. 
In  teeth  with  very  flat,  low  cusps  the  closure  of  the  grooves  may  be 
very  perfect,  leaving  only  a  slight  depression  (Fig.  91). 


120  STRUCTURAL  DEFECTS  IN  THE  ENAMEL 

The  importance  of  these  defects  as  positions  of  beginning  caries 
cannot  be  overestimated,  as  they  furnish  ideal  conditions  in  areas 
that  would  otherwise  be  immune,  and  they  are  the  positions  in 
which  the  attacks  of  caries  are  first  manifested.  These  occlusal 
grooves  appear  in  great  variety.  Some  are  simply  shallow,  open 
grooves,  in  which  the  surface  of  the  enamel  is  perfect  (Fig.  88) ; 
some  are  very  deep  and  entirely  empty  (Figs.  89,  90,  and  92); 
others  are  apparently  filled  with  a  granular,  more  or  less  structure- 
less calcified  material  which  appears  to  have  been  deposited  in 
the  groove  after  the  enamel  was  completed  (Figs.  93,  98,  and  99). 
This  is  probably  of  the  nature  of  cementum.  It  was  formed  after 


FIG.  98. — An  occlusal  defect  in  a  worn  tooth.    The  fissure  is  filled  with  coronal 

cementum. 


the  enamel  was  completed,  but  while  the  tooth  was  enclosed  in 
its  follicle  in  the  crypt  in  the  bone.  It  is  to  be  compared  with  the 
coronal  cementum  that  is  characteristic  of  the  complex  grinding 
teeth  of  the  ungulates  and  other  herbivorous  animals.  A  study 
of  these  defects  furnishes  the  basis  for  the  operative  rule  that  "all 
grooves  must  be  cut  out  to  the  point  where  the  margin  will  be  on 
a  smooth  surface."  For  if  they  are  not,  a  defect  will  be  left  at  the 
margin  of  the  cavity  which  offers  ideal  conditions  for  the  beginning 
of  a  new  decay.  When  caries  begins  in  such  a  defect  at  the  margin 
of  a  filling,  it  progresses  at  the  bottom  of  the  defect  until  the  dento- 
enamel  junction  is  reached,  and  then  extends  in  the  dentin  and 
may  destroy  the  entire  crown  without  showing  upon  the  surface 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


121 


(page  50).  The  extent  of  these  defects  is  much  greater  than 
would  be  supposed  from  the  observation  of  the  teeth  in  the 
mouth.  Fig.  94  shows  the  occlusal  view  of  a  lower  third  molar, 


FIG.  99. — Higher  magnification  of  Fig.  98.     The  fissure  filled  with  granular  calcified 
material.     Notice  the  direction  of  the  bands  of  Ret/ius  around  the  fissure. 

extracted  because  of  disease  of  the  peridental  membrane,  from  a 
man  aged  about  forty  years.  Examining  these  grooves  with  a 
fine-pointed  explorer,  it  would  not  stick  any  place.  No  operator 


122 


STRUCTURAL  DEFECTS  IN   THE  ENAMEL 


would  think  of  cutting  them  out  and  filling  them.  The  crown  was 
sawed  through  from  buccal  to  lingual,  as  shown  in  Fig.  95,  and  the 
piece  marked  1  is  shown  in  Figs.  96  and  97.  The  grooves  are  open 


E 


1  V&X&S& 


""..  ?<:^3ss:?£^??r-' ' 


FIG.  100. — Structural  defects  in  developmenta    grooves  on  axia    surfaces      (Black.) 

two-thirds  of  the  distance  to  the  dento-enamel  junction,  and  show 
slight  action  of  caries.  Suppose  caries  had  started  in  the  central 
pit,  and  a  small  round  filling  had  been  made,  open  defects  would 


FIG.  101. — Structural  defects  in  developmental  grooves  on  axial  surfaces.     (Black.) 

be  left  at  the  margin  where  every  groove  radiated  from  the  central 
cavity,  and  these  would  be  just  as  liable  to  recurrent  decay  as 
they  were  originally,  and,  if  caries  occurred,  it  would  progress  at 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL  123 

the  depth  of  the  groove,  reach  the  dento-enamel  junction,  and 
progress  in  the  dentin,  until  the  occlusal  enamel  was  so  undermined 
that  it  would  break  in  under  the  force  of  mastication.  On  the 
other  hand,  if  the  grooves  are  cut  out  to  a  point  where  the  cavity 
margin  will  be  on  a  smooth  surface,  there  is  no  possibility  of  recur- 
rent caries  if  the  filling  material  is  properly  inserted.  This  one 
illustration,  which  might  be  duplicated  a  thousand  times,  there- 
fore is  the  rational  basis  for  the  rule,  "All  grooves  must  be  cut  out 
to  their  ends." 

Caries  does  not  occur  in  all  open  grooves.     Fig.  90  shows  an 
open  groove  in  a  section  from  a  tooth  in  which  the  wear  indicates 


FIG.  102. — Defects  on  the  axial  surface  in  the  enamel. 

that  it  was  not  from  a  young  person,  but  most  of  the  grooves  that 
escape  are  not  open,  but  more  or  less  entirely  filled  with  structure- 
less calcified  matter  or  coronal  cementum.  Figs.  93,  98,  and  99 
are  very  good  illustrations  of  this  class  of  grooves. 

The  condition  in  pits  from  which  grooves  extend,  as  the  lingual 
pits  of  incisors  and  the  buccal  pits  of  molars,  show  the  same  con- 
dition as  the  grooves,  except  that  the  defect  is  both  broader  and 
deeper.  But  pits  that  are  sometimes  found  on  the  tips  of  cusps 
and  on  smooth  surfaces  show  an  entirely  different  structural  con- 
dition, and  are  pathologic  in  character. 

In  places  where  the  union  of  the  enamel  plates  seems  perfect, 


124 


STRUCTURAL  DEFECTS  IN  THE  ENAMEL 


as,  for  instance,  on  the  labial  surface  of  the  incisors  or  the  buccal 
surface  of  the  bicuspids,  and  the  line  of  union  is  marked  only  by 
a  slight  depression  of  the  surface,  the  section  will  show  disturbance 
of  structure.  Fig.  101,  a  drawing  made  by  Dr.  Black  a  good  many 
years  ago,  shows  such  a  position.  At  the  surface  the  rods  and 
their  arrangement  seem  very  perfect,  but  from  a  point  about  one- 
third  the  distance  to  the  dento-enamel  junction  there  are  no  rods 
at  all,  but  apparently  a  number  of  calcospherites  in  a  granular 


FIG.  103. — A  section  through  such  a  defect  as  that  shown  in  Fig.  102.    (About  SO   X) 


calcific  substance.  In  Fig.  101,  another  of  Dr.  Black's  illustrations, 
the  rods  are  very  irregular,  and  are  separated  by  large  areas  of 
structureless  calcified  material.  Grooves  are  often  found  in  unusual 
or  atypical  positions.  Fig.  102  shows  a  groove  running  over  the 
mesial  marginal  ridge  and  down  on  the  mesial  surface.  Fig.  103 
shows  a  section  through  such  a  defect.  Notice  the  folding  of  the 
enamel  into  the  dentin  and  the  disturbance  of  the  rods  about 
the  groove  and  between  its  base  and  the  dentin. 


CHAPTER  X. 

SPECIAL   AREAS   OF  WEAKNESS   FOR  ENAMEL 
MARGINS. 

THERE  are  certain  positions  which  in  the  perfect  crown  are 
areas  of  great  strength,  but  which,  because  of  the  peculiar  structure 
of  the  tissue  in  these  places,  become  areas  of  weakness  when  cavity 


FIG.  104  — Buccolingual  section  of  upper  bicuspid.     Enamel  is  broken  from  grinding. 
A  to  B,  area  of  weakness  for  enamel  margin.     (About  20  X) 

(125) 


126         AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 

margins  are  made  in  them.  The  treatment  of  beginning  caries 
would  lead  to  no  failures  in  these  positions,  for  cavity  margins 
would  never  be  extended  into  them,  except  in  the  treatment  of 


FIG.  105. — Enamel  over  tip  of  dentin  cusp:  D,  dentin  cusp.     (About  80  X).    From 
same  section  as  Fig.  104 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


127 


burrowing  caries  and  neglected  cases.  The  extension  of  caries 
at  the  dento-enamel  junction  often  requires  the  extension  of  the 
margin  into  the  area  of  danger.  In  considering  these  areas  and  in 
the  preparation  of  cavities,  as  well  as  the  areas  of  imperfect  struct- 
ure considered  in  Chapter  IX,  it  is  important  to  place  as  much 
emphasis  on  the  necessity  of  not  extending  cavity  margins  into  the 
areas  of  weakness,  as  on  cutting  away  the  dangerous  area  and  leaving 
the  margin  in  a  safe  position,  when  the  area  cannot  be  avoided. 

In  considering  the  relation  of  the  enamel  and  dentin,  and  in 
studying  the  arrangement  of  the  enamel-rod  direction  in  the 
"architecture"  of  the  tooth  crown, 
it  has  been  pointed  out  that  the 
dentin  cusps  and  the  dentinal 
marginal  ridges  are  not  directly 
under  the  corresponding  points  on 
the  surface  of  the  enamel,  but  are 
nearer  to  the  axis  of  the  tooth. 
The  areas  on  the  surface  of  the 
enamel,  from  the  point  directly 
Over  the  tip  of  the  dentin  cusp 
or  ridge  to  the  tip  of  the  enamel 
cusps  or  ridges,  become  areas  of 
weakness  when  a  cavity  is  ex- 
tended into  them. 

Fig.  104  is  a  photomicrograph 
of  a  .buccolingual  section  of  a 
superior  bicuspid,  and  Fig.  105  is 
a  higher  magnification  of  the  same, 
made  to  illustrate  the  condition. 
It  will  be  seen  that  if  decay  has 

extended  at  the  dento-enamel  junction  to  the  tip  of  the  dentin 
cusp,  and  the  enamel  walls  were  left  in  the  axial  plane,  the  rods 
which  form  the  surface  of  the  enamel  from  the  margin  of  the  cavity 
to  the  tip  of  the  cusp  "are  not  supported  by  dentin,"  and  would  be 
likely  to  be  broken  and  fall  away,  leaving  a  defect  at  the  margin 
of  the  filling.  If  decay  beginning  in  the  groove  or  pit  has  extended 
only  to  point  C,  Fig.  104,  the  wall  may  be  trimmed  in  the  axial 
plane  and  an  ideal  wall  produced;  but  if  it  has  reached  point  D, 
Fig.  104,  it  must  be  inclined  buccally  so  as  to  remove  the  tip  of  the 
cusp,  as  indicated  in  the  dotted  line,  and  the  cusp  restored  by  the 
filling  material.  The  region  of  the  surface  indicated  by  A-B, 


FIG.  106. — A  bicuspid  cut  for  sec- 
tioning. Sections  were  ground  from 
the  positions  marked  by  the  lines 
1,  2,  3,  4,  and  4  is  also  shown  in 
Figs.  107,  and  108. 


128 


AREAS  OF  WEAKNESS  FOR  ENAMEL   MARGINS 


while  an  area  of  strength  in  the  perfect  tissue,  becomes  a  position 
of  weakness  when  cavity  margins  are  extended  into  it.     A  careful 


FIG.  107. — Section  ground  from  Fig.  106,  through  mesial  part  and  marginal  ridge. 
If  caries  has  extended  at  the  dento-enamel  junction  to  A,  the  wall  may  be  in  the 
axial  plane;  if  it  has  reached  B,  the  wall  must  be  inclined  as  indicated  by  the  dotted 
line.  (About  30  X) 


observer  will  find  many  failures  that  are  the  result  of  bad  enamel- 
wall  preparation  in  these  areas.    The  same  conditions  exist  in  the 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


129 


region  of  the  marginal  ridges.  Figs.  107  and  108  show  the  mesial 
marginal  ridge  of  a  superior  bicuspid.  If  this  is  filled  before  the 
destruction  of  dentin  has  extended  beyond  the  point  A,  the  mesial 
wall  may  be  cut  in  the  axial  plane  as  indicated;  but  if  it  has  reached 
the  tip  of  the  dentin  ridge  at  point  B,  it  must  be  inclined  mesially, 
so  as  to  reach  the  tip  of  the  enamel  ridge.  Figs.  109  and  '110  show 


FIG.  108. — A  higher  magnification  of  Fig.  107,  showing  enamel-rod  directions  in  the 
region  of  the  marginal  ridge. 

the  distal  marginal  ridge  in  a  second  molar.  Notice  the  inclination 
of  the  rods  from  the  tip  of  the  dentin  ridge.  If  decay  has  reached 
this  point  the  wall  must  be  inclined  distally,  so  as  to  reach  the  rod 
direction,  or  a  frail  margin  will  be  left  and  one  which  will  not  sus- 
tain the  force  of  mastication.  Neglected  caries  in  the  lingual  pits 
of  incisors  often  present  the  same  conditions  as  found  in  the  mar- 

9 


130 


AREAS  OF  WEAKNESS  FOR  ENAMEL   MARGINS 


ginal  ridges  of  the  occlusal  surface  of  molars  and  bicuspids.    The 
same  conditions  are  also  often  encountered  in  the  preparation  of 
simple  cavities  in  the  mesial  or  distal  surfaces  of  incisors,  when 
caries  has  followed  the  dento-enamel  junc- 
tion toward  the  lingual.     Fig.  Ill  shows  a 
superior  central  incisor  from  which  sections 
•'  were    cut  as  indicated.    Suppose  caries    to 
have  begun    in    the   region  of   the  contact 
point  and   to   have  extended   to  the  point 
a.     If  the    lingual  enamel   wall   were   pre- 
FIG.   109.— An    upper  pared  at  the  line  A,  Fig.  112,  a  very  frail 
molar  showing  the  posi-  wan  would  result.    Force  coming  upon  this 

tion  of  the  section  shown  .  . 

in  Fig.  no.  wall  from  the  lingual  by  the  occlusion  of  the 

lower  incisors,  would  be  likely  to  break  out 

or  crack  a  triangular  piece  of  enamel,  and  the  filling  would  fail 
along  the  lingual  wall.  If,  however,  the  wall  be  laid  in  the  line  at 
B,  a  strong  wall  is  produced,  against  which  gold  can  be  properly 
condensed  without  danger,  and  which  will  withstand  the  force  of 
occlusion. 


FIG.  110. — The  section  ground  from  Fig.  109. 


Dentists  are  often  tempted  to  prepare  simple  cavities  in  the 
mesial  surfaces  of  first  and  second  bicuspids  and  occasionally  in 
the  molars.  If  this  is  ever  done,  it  must  be  with  the  full  knowledge 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


131 


both  of  the  liability  of  recurrence  of  caries  and  the  structure  of 
the  enamel,  for  experience  shows  that  such  operations  usually  fail, 
either  by  recurrence  of  caries  at  the  bucco- 
gingival  or  linguogingival  angles,  or  by  the 
breaking  out  of  the  enamel  of  the  marginal 
Bridge.     Fig.  115  shows  the  mesial  surface  of 
a  superior  bicuspid.    There  was  a  white  spot 
on  the  contact  point,  but  no  actual  cavity, 
as  the  enamel  rods   had  not  fallen  out.    A 
section  was  ground  through  this  point,  and 
Fig.   116    shows    a    photomicrograph   of    it. 
The    enamel    rods    have    fallen    out  of    the 
disintegrated    area,    and    the    decalcification 
in  the    dentin    is  shown.     If   this    had  been 
treated  as  a  simple  cavity  the  occlusal  wall 
would  have  required  an  inclination  of  18  centigrades  occlusally 
from   the  horizontal   plane  to   reach    the    enamel-rod    direction. 
There  is  very  little  support  offered  by  the  dentin  for  the  enamel 
of  the  marginal  ridge,  and  the  portion  over  to  the  occlusal  groove 
would  be  likely  to  be  broken  off  by  the  force  of  mastication.    The 
conditions  of  the  occlusal  wall  are  better  shown  in  Fig.  117. 


FIG.  1 1 1. — A  superior 
central  incisor,  showing 
the  position  of  sections 
in  Figs.  112,  113  and  114. 


FIG.  112. — Section  1,  Fig.  Ill,  showing  the  enamel  worn  from  the  marginal  ridges. 

Any  number  of  illustrations  of  these  conditions  might  be  made, 
but  the  subject  may  be  summed  up  by  saying:  The  surface  of  the 
enamel  from  the  point  directly  over  the  dentin  cusp  or  ridge  to  the 
tip  of  the  enamel  cusp  or  ridge,  which  is  an  area  of  great  strength 
in  the  perfect  crown,  is  a  region  of  weakness  for  an  enamel  wall. 
It  is  fully  as  important  not  to  extend  into  this  area  unnecessarily 


132         AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


FIG.  113. — Section  2,  Fig.  Ill,  showing  position  of  weak  and  strong  lingual  walls. 


FIG.  114. — A  higher  magnification  of  the  mesial  marginal  ridge,  shown  in  Fjg.  113. 

(About  60  X) 


AREAS  OF   WEAKNESS  FOR  ENAMEL  MARGINS 


133 


as  to  form  the  wall  proper  when  caries  has  extended  so  as  to 
involve  it.  When  caries  of  a  smooth  surface  approaches  a  mar- 
ginal ridge  which  receives  the  force  of  occlusion,  the  wall  must 


FIG.  115. — Occlusal  and  mesial  views  of  a  superior  bicuspid,  showing  position  of 
section.  A  beginning  caries  could  be  seen  on  the  surface,  but  it  does  not  show  well 
in  the  picture.  The  section  from  the  buccal  piece  is  shown  in  the  following  illustra- 
tions. 


FIG.  116. — The  section  ground  from  the  buccal  piece,  Fig.  115. 


134 


AREAS  OF  WEAKNESS  FOR  ENAMEL  MARGINS 


be  extended  so  that  the  enamel  receives  full  support  from  sound 
dentin. 


FIG.  117. — The  enamel  over  the  mesial  marginal  ridge  to  the  oblique  groove,  show- 
ing a  region  of  weakness  for  the  occlusal  wall  of  a  simple  proximal  cavity. 


CHAPTER  XI. 
THE  DENTIN. 

THE  dentin  may  be  defined  as  a  connective  tissue  whose  inter- 
cellular substance  is  calcified.  It  is  apparently  homogeneous  in 
structure,  but  penetrated  by  minute  canals,  which  contain  proto- 
plasmic projections  from  cells  lying  within  a  cavity  enclosed  by 
the  tissue. 

The  Function  of  the  Dentin. — The  dentin  makes  up  the  mass  of 
the  tooth,  giving  to  it  its  general  form,  each  cusp  and  root  being 
indicated  in  it.  It  gives  to  the  tooth  its  elastic  strength,  and  the 
enamel,  being  hard  and  very  resistant  to  abrasion  but  extremely 
brittle,  is  dependent  upon  the  elastic  support  of  the  dentin.  This 
has  been  elaborated  to  a  considerable  extent  in  the  chapter  on  the 
Dental  Tissues.  The  fact  that  the  dentin  gives  the  strength  to 
the  tooth  should  never  be  lost  sight  of  in  operating,  and  sound 
dentin  should  never  be  sacrificed  unnecessarily  in  the  preparation 
of  cavities. 

Structural  Elements  of  the  Dentin. — The  structural  elements  of 
the  dentin  may  be  stated  as : 

1.  The  dentin  matrix. 

2.  The  sheaths  of  Newman  and  the  dentinal  tubules. 

3.  The  contents  of  the  dentinal  tubules  or  the  dentinal  fibrils. 
While  these  are  the  elements  of  which  the  tissue  is  composed, 

there  are  other  characteristic  appearances  found  in  the  dentin, 
caused  by  special  conditions  or  arrangement  of  these  elements 
which  must  be  studied.  These  are  the  granular  layer  of  Tomes, 
the  interglobular  spaces,  the  lines  of  Schreger,  and  secondary 
dentin. 

Origin  of  the  Tissue  (Histogenesis). — The  dentin,  like  all  of  the 
other  calcified  tissues  except  the  enamel,  is  a  connective  tissue,  and 
is  formed  by  the  dental  papilla,  which  is  a  conical  papilla  of  con- 
nective tissue  rich  in  bloodvessels  and  covered  on  its  surface  by 
the  layer  of  dentin-forming  cells,  the  odontoblasts.  The  dentin 
is  formed  from  without  inward,  leaving  the  remains  of  the  dental 
papilla  in  the  cavity  of  the  formed  dentin  as  the  dental  pulp.  Before 

(135) 


136  THE  DENT  IN 

the  tooth  is  erupted,  and  up  to  the  time  that  the  full  length  of  the 
root  is  formed,  a  characteristic  thickness  of  dentin  is  formed,  which 
is  called  the  primary  dentin.  After  this  time  dentin  is  formed  by 
the  pulp  only  intermittently,  in  response  to  irritations  and  trophic 
impulses,  producing  secondary  dentin.  Secondary  dentin  is  always 
more  irregular  in  the  arrangement  of  the  tubules,  and  more  imper- 
fect in  structure  than  the  primary  dentin.  The  boundary  line 
between  two  periods  of  dentin  formation  can  always  be  picked  out 
by  changes  in  the  direction  or  character  of  the  dentinal  tubules. 

The  Dentin  Matrix. — The  dentin  matrix  is  a  solid,  apparently 
homogeneous,  and  very  elastic  substance,  through  which  the  den- 
tinal tubules  extend.  It  is  translucent  in  appearance  and  slightly 
yellowish  in  color.  In  broken  or  split  sections  to  the  unaided  eye 
it  has  a  yellowish  color  by  reflected  light,  and  a  characteristic 
luster  due  to  the  refraction  of  light  by  the  tubules.  In  ground 
sections,  by  transmitted  light,  under  the  microscope,  it  is  very 
translucent  and  shows  no  indication  of  structure. 

The  matrix  consists  of  an  organic  basis  of  ultimately  fibrous 
character,  yielding  gelatin  on  boiling,  with  which  the  inorganic 
salts  are  chemically  combined.  The  relation  of  organic  and  inor- 
ganic matter  in  the  dentin  matrix  is  similar  to  the  condition  in 
the  bone  matrix  and  that  of  all  calcified  connective  tissues.  Appar- 
ently the  organic  basis  is  first  formed,  and  then  the  inorganic 
salts  are  combined  with  it  in  a  weak  chemical  union.  If  the  dentin 
is  treated  with  dilute  acid,  the  inorganic  matter  is  dissolved  and 
the  organic  basis  is  left  retaining  the  form  of  the  tissue.  If  the 
organic  matter  is  burned  out,  it  leaves  the  inorganic  matter  in  the 
characteristic  form. 

Von  Bibra  gives  the  following  analysis  of  perfectly  dry  dentin : 

Organic  matter 27.61 

Fat 0.40 

Calcium  phosphate  and  fluoride 66.72 

Calcium  carbonate 3.36 

Magnesium  phosphate 1 . 08 

Other  salts 0.83 

Mr.  Charles  Tomes  pointed  out  that  such  analyses  as  this  failed 
to  take  account  of  about  8  per  cent,  of  water  which  is  held  as  water 
of  combination,  and  which  is  driven  off  at  about  red  heat. 

It  is  evident  that  the  organic  matter  in  the  dentin  is  of  two  kinds 
—the  organic  basis  of  the  matrix,  which  is  of  gelatin-yielding 
character,  and  the  protoplasmic  contents  of  the  dentinal  tubules. 
Variations,  therefore,  in  the  proportion  of  organic  and  inorganic 


THE  SHEATHS  OF  NEWMAN  137 

matter  in  the  dentin  might  be  caused  by  differences  in  the  propor- 
tions of  organic  and  inorganic  constituents  of  the  matrix,  or  by 
variations  in  the  size  of  the  tubules  and  the  amount  of  material 
contained  in  them. 

If  dentin  changes  in  its  degree  of  calcification  with  age,  this 
might  be  brought  about  by  the  reduction  in  the  size  of  the  tubules, 
or  by  the  adding  of  inorganic  constituents  to  the  matrix. 

The  amount  of  material  contained  in  the  dentinal  tubules  is 
much  greater  than  is  generally  realized.  If  2^  is  considered  the 
average  diameter  of  the  dentinal  tubules,  and  they  are  separated 
by  an  average  of  SM  of  dentin  matrix.  Some  idea  of  the  relative 
volume  of  the  dentin  matrix  and  the  contents  of  the  tubules  can  be 
obtained,  but  this  is  greatly  increased  by  the  very  numerous  side 
branches  which  connect  the  neighboring  tubules.  This  matter  can 
be  visualized  by  taking  a  lump  of  soft  clay  and  boring  it  full  of  holes, 
making  the  holes  two  inches  in  diameter,  and  separated  by  eight 
inches  of  clay. 

The  ultimately  fibrous  character  of  the  dentin  matrix  can  be 
made  out  only  in  various  stages  of  decalcification  and  decomposi- 
tion. In  the  original  condition  no  trace  of  the  fibrous  character 
can  be  seen.  By  maceration  with  acids  and  alkalies  the  intertubular 
material  assumes  a  fibrous  appearance,  as  if  bundles  of  white  con- 
nective-tissue fibers  had  been  fused  together.  There  is  apparently 
no  definite  arrangement  of  these  fibers  and  there  is  no  indication 
of  the  arrangement  of  the  substance  in  layers. 

The  Sheaths  of  Newman. — There  has  been  much  discussion  as 
to  the  character  of  these  structures,  which  were  first  discovered 
in  1863  by  Newman.  Some  investigators  have  denied  their  exist- 
ence entirely,  explaining  the  appearance  in  some  other  way.  These 
structures  are  in  no  sense  a  sheath  surrounding  the  dentinal  fibril 
and  lying  in  the  dentinal  tubule,  but  are  that  portion  of  the  matrix 
which  forms  the  immediate  wall  of  the  tubule.  That  this  material 
differs  from  that  which  occupies  the  rest  of  the  space  between  the 
tubules  is  certain,  and  is  shown  by  the  examination  of  ground  sec- 
tions, the  action  of  stains  upon  ground  sections,  and  the  action  of 
the  matrix  when  boiled  with  strong  acids  and  alkalies.  In  Fig. 
118,  a  photograph  of  a  ground  section,  there  is  evidently  a  dif- 
ference in  the  refracting  index  of  the  portion  of  the  matrix  imme- 
diately surrounding  the  tubules.  Apparently  the  sheaths  of  New- 
man are  composed  of  a  material  similar  to  that  forming  elastic 
connective-tissue  fibers,  and  known  as  elastin.  This  substance 


138  THE  DEN  TIN 

is  very  resistant  to  the  action  of  acids  and  alkalies.  After  the 
remainder  of  the  intertubular  material  has  been  destroyed  by 
boiling  with  strong  acid,  the  sheaths  remain  like  hollow  elastic 
fibers,  having  the  appearance  of  pipe-stems,  which  resist  long-con- 
tinued action  of  the  boiling  acid.  Some  authors  have  suggested 
that  the  great  elasticity  of  the  dentin  was  largely  due  to  the  presence 
of  this  substance.1 


Fid.  118. — Dentin  showing  tubules  in  cross-section:    Dt,  dentinal  tubules;  D,  dentin 
matrix;  S,  shadow  of  sheaths  of  Newman.     (About  1150  X) 

The  Dentinal  Tubules. — The  dentin  matrix  is  penetrated  every- 
where by  minute  branching  tubules,  which  radiate  from  the  central 
cavity  or  pulp  chamber  and  extend  to  the  outer  surface  of  the 
dentin  at  the  dento-enamel  junction  or  the  dento-cemental  junction, 
where  they  end  blindly  or  in  irregular  enlargements.  These  tubules 
are  from  1.1  to  3  microns  in  diameter.  One  hundred  measurements2 
made  at  random  from  ground  sections  gave  the  extreme  measure- 
ment: 3,  largest;  1.5,  smallest;  and  average,  2.95.  Fifty  measure- 
ments from  one  longitudinal  section  of  tubules  at  their  pulpal 
extremity  gave  an  average  of  2.6;  largest,  3;  smallest,  1.5;  and 
50  measurements  at  the  dento-enamel  junction  of  the  same  section 

1  Hawazawa,  Tokyo:  A  Study  of  the  Minute  Structure  of  Human  Dentin,  Trans. 
Panama  Pacific  Dental  Congress,   1915,  p.  80,  and  Dental  Cosmos,  February  and 
March,  1917,  vol.  ix. 

2  Kolliker  gives  5  microns,  also  Schafer;    Owen,  2.5  microns. 


DIRECTION  OF  TUBULES  IN  CROWN  PORTION 


139 


gave  the  following:  Average,  1.2;  largest,  1.5;  smallest,  0.75. 
These  measurements  were  made  with  an  eye-piece  micrometer, 
using  yg-  oil-immersion  objective  and  No.  3  ocular. 

At  the  present  time  there  is  a  fertile  field  for  investigation  offered 
in  regard  to  the  size  of  dentinal  tubules.  Many  statements  have 
been  made  that  have  not  been  supported  by  tabulated  measure- 
ments, and  no  definite  statement  can  be  made  as  to  the  variations 
and  size  of  the  dentinal  tubules  in  different  teeth,  the  teeth  of 
different  animals,  or  in  the  human  teeth  at  different  ages. 


FIG.  119. — A  section  showing  the  primary  curvatures  of  the  dentinal  tubules  in  the 
crown  portion.    (About  20  X) 

Direction  of  Tubules  in  Crown  Portion. — In  the  crown  portion  and 
the  gingival  portion  of  the  dentin  the  tubules  pass  from  the  pulp 
chamber  to  the  dento-enamel  junction,  or  the  dento-cemental 
junction,  in  sweeping  curves,  which  were  called  by  Tomes  the 
primary  curvatures.  These  have  been  described  as  /-  or  ^-shaped 
(Fig.  119).  The  tubule  tends  to  enter  the  pulp  chamber  at  right 
angles  to  the  surface,  and  to  end  at  the  dento-enamel  junction  at 
right  angles  to  that  surface.  In  the  dentin  forming  the  axial  walls 
of  the  pulp  chamber  the  tubules  make  two  bends  in  passing  from 
the  pulp  chamber  to  the  surface  of  the  dentin.  In  the  first  the  con- 
vexity is  directed  apically,  in  the  second  it  is  directed  occlusally. 


140 


THE  DEN  TIN 


The  outer  extremity  of  the  tubule  is  therefore  considerably  farther 
to  the  occlusal  than  the  point  at  which  it  opens  into  the  pulp  cham- 


FIG.  120. — A  section  showing  the  primary  curvature  of  the  dentinal  tubules  in  the 
gingival  portion.     (About  20  X) 


FIG.  121. — A  section  showing  compound  curves  near  the  dento-enamel  junction. 

(About  80  X) 


DIRECTION  OF  TUBULES  IN  CROWN  PORTION 


141 


her  (Fig.  120).  The  outer  part  of  this  double  curve  is  often  complex 
instead  of  simple  (Fig.  121).  The  course  of  the  dentinal  tubules 
is  not  a  direct  one,  but  that  of  an  open  spiral.  This  may  easily 
be  demonstrated  by  changing  the  focus  up  and  down  in  examining 
sections  cut  at  right  angles  to  the  direction  of  the  tubules.  When 
examined  in  longitudinal  sections  this  spiral  course  gives  to  the 


FIG.    122. — Dentin  at  dento-enamel   junction,   showing  tubules  cut  longitudinally 

(About  760  X) 


tubule  the  appearance  of  having  little  wavy  curves  throughout 
its  length.  These  have  often  been  called  the  secondary  curvatures. 
Each  wave  represents  a  turn  in  the  spiral.  As  many  as  two  hun- 
dred have  been  counted  in  the  length  of  a  single  tubule,  or  about 
one  hundred  in  a  millimeter  of  length. 

The  dentinal  tubules  give  off  minute  lateral  branches,  which 
extend  from  one  tubule  to  another.     These  are  very  minute,  and 


142 


THE  DENTIN 


in  the  crown  portion  of  the  dentin  are  not  at  all  conspicuous,  but 
in  the  region  of  the  dento-enamel  junction  the  tubules  branch 


FIG.   123. — Dentin  from  the  root,  showing  tubules  cut  longitudinally  and   the  fine 
connecting  branches.     (About  700  X) 


DENTINAL  TUBULES  IN  THE  ROOT  PORTION 


143 


dichotomously,  each  fork  having  about  the  same  diameter  as  the 
original  tubule  (Fig.  122).  These  forkings  of  the  tubules  resemble 
the  appearance  of  the  delta  of  a  river  on  the  map.  The  branches 
anastomose  with  each  other  very  freely.  This  anastomosis  of  the 
tubules  at  the  dento-enamel  junction  is  very  important  in  deter- 
mining the  spreading  of  caries  in  this  area.  It  probably  also  explains 
the  sensitiveness  of  this  area  noticed  in  the  preparation  of  cavities, 
which  will  be  noted  again  in  considering  the  sensitiveness  of  the 
dentin. 


FIG.  124. — Granular  layer  of  Tomes:  L,  lacunae  of  cementum;   GT,  granular  layer  of 
Tomes;   Ig,  interglobular  spaces.     (About  200  X) 


The  Dentinal  Tubules  in  the  Root  Portion. — In  the  root  portion 
of  the  dentin  the  tubules  ordinarily  show  only  the  secondary  curves, 
their  general  direction  being  at  right  angles  to  the  axis  of  the  pulp 
canal.  Throughout  their  course  they  give  off  an  enormous  number 
of  very  fine  branches  extending  from  tubule  to  tubule.  These  are 


144  THE  DENTIN 

so  numerous  that  in  suitably  prepared  sections  they  may  be  said 
to  look  like  the  interlacing  twigs  of  a  thicket  or  the  rootlets  of 
plants  in  the  soil.  Fig.  123  gives  a  very  good  idea  of  the  appear- 
ance. 

At  the  dento-cemental  junction  the  tubules  end  in  irregular 
anastomosing  spaces,  which  cause  the  appearance  of  the  granular 
layer  of  Tomes  (Fig.  124). 

From  a  consideration  of  the  preceding  it  will  be  seen  that  it  is 
usually  not  difficult  to  determine  whether  a  field  of  dentin  seen 
under  the  microscope  was  taken  from  the  crown  or  the  root  of  a 
tooth.  The  structural  characteristics  of  the  two  regions  may  be 
summarized  as  follows:  In  the  crown,  the  tubules  show  both  the 
primary  and  the  secondary  curves.  In  the  root,  the  tubules  show 
only  the  secondary  curves.  In  the  crown,  the  lateral  branches  are 
few  and  inconspicuous  and  the  tubules  branch  in  a  characteristic 
way  at  the  dento-enamel  junction.  In  the  root,  the  lateral  branches 
are  very  numerous  throughout  the  length  of  the  tubule,  and  they 
end  in  the  characteristic  spaces  of  the  granular  layer  of  Tomes. 

The  Dentinal  Fibrils. — In  life  the  dentinal  tubules  are  occupied 
by  protoplasmic  projections  of  the  odontoblasts  known  as  the 
dentinal  fibrils,  or  fibers  of  Tomes.  As  the  dentin  matrix  is  formed 
and  calcified  under  the  influence  of  the  odontoblasts,  a  portion  of 
their  protoplasm  is  left  in  the  tubules  of  the  matrix  as  the  dentinal 
fibril.  These  structures  were  first  described  by  John  Tomes,  who 
recognized  their  true  character.  They  may  be  demonstrated  in 
decalcified  sections,  and  they  will  be  seen  projecting  from  the 
odontoblasts,  when  the  pulp  is  removed  from  a  freshly  extracted 
tooth,  by  cracking  it  and  picking  the  pulp  out.  In  this  way  a 
portion  of  the  fibril  is  pulled  out  of  the  tubules.  The  fibrils  will  be 
considered  more  especially  in  connection  with  the  pulp,  to  which 
they  properly  belong. 

In  the  author's  opinion  very  little  is  positively  known  about  the 
contents  of  the  dentinal  tubules.  While  it  is  very  apparent  that  in 
young  forming  dentin  the  tubules  are  filled  by  cytoplasmic  projec- 
tions of  the  odontoblasts,  it  is  by  no  means  certain  that  all  of  the 
tubules  of  the  dentin,  in  an  old  tooth  are  still  occupied  by  living 
cytoplasm.  What  the  fate  of  the  cytoplasmic  contents  of  the  tubules 
is  when  secondary  dentin  is  formed  is  not  known.  Several  things 
are  theoretically  possible  but  there  is  little  or  no  direct  evidence 
on  the  matter. 


THE  GRANULAR  LAYER  OF  TOMES  145 

The  Granular  Layer  of  Tomes. — The  granular  layer  of  Tomes  is 
the  outer  layer  of  the  dentin  next  to  the  cementum.  The  granular 
appearance  is  caused  by  irregular  spaces  in  the  dentin  matrix  which 
connect  with  the  ends  of  the  dentinal  tubules,  and  which  are  filled 
with  protoplasm  continuous  with  that  of  the  fibrils.  Tomes  first 
called  attention  to  this  layer,  and  for  this  reason  it  bears  his  name. 

With  magnifications  of  from  50  to  100  diameter  it  is  easily  seen 
in  ground  sections,  either  longitudinal  or  transverse,  and  appears 
as  a  layer  filled  with  little  dark  spots  or  granules,  the  spaces  which 
have  been  filled  with  the  debris  of  grinding.  It  is  separated  from 
the  cementum  by  a  thin,  clear  layer,  apparently  of  structureless 
dentin  matrix,  which  is  more  apparent  in  higher  magnifications. 
The  granular  layer  is  sometimes  seen  in  the  crown  portion  just 
under  the  enamel,  but  it  is  never  as  well  marked  in  this  position. 

The  layer  is  seen  in  sections  ground  from  freshly  extracted  teeth 
as  well  as  from  old  dry  teeth,  showing  that  these  are  true  spaces 
and  are  not  produced  by  the  shrinkage  of  partially  calcified  dentin 
matrix.  Tomes  called  the  spaces  in  the  granular  layer  "inter- 
globular  spaces,"  but  this  term  should  not  be  used,  as  the  structures 
generally  known  as  the  interglobular  spaces  are  different  in  location 
and  character,  and  will  be  considered  later. 

The  granular  layer  is  not  seen  in  decalcified  sections.  So  far  as 
the  author  is  aware,  no  one  has  called  attention  to  this  fact  before. 
In  decalcified  sections  stained  with  hematoxylin  and  eosin  the 
position  of  the  granular  layer  is  always  occupied  by  a  clear  layer 
which  takes  the  stain  in  an  entirely  different  wray  from  the  rest  of 
the  dentin  matrix,  and  in  which  no  indication  of  spaces  can  be 
seen.  \Yhile  the  fibrils  in  the  tubules  through  most  of  the  dentin 
take  the  hematoxylin  stain  and  can  be  easily  seen,  they  cannot  be 
followed  into  this  clear  layer,  and  no  indication  of  protoplasmic 
contents  of  irregular  spaces  can  be  seen.1 

Dr.  Skillen  has  worked  out  a  method  of  demonstrating  the 
granular  layer  of  Tomes  in  decalcified  sections  which  is  reported  in 
an  article  by  Dr.  Newton  G.  Thomas  in  the  Dental  Cosmos  for 
June,  1920. 

Most  authorities  state  that  the  spaces  of  the  granular  layer 
communicate  with  the  canaliculi  of  the  cementum,  as  well  as  with 
the  tubules  of  the  dentin.  This  the  author  has  been  unable  to 

1  The  appearance  of  the  tissue  in  decalcified  sections  had  led  to  some  doubt  in  the 
writer's  mind  as  to  the  interpretation  of  the  character  of  the  layer  by  authors  who 
have  described  it. 

10 


146  THE  DENTIN 

confirm.  On  the  other  hand,  the  granular  layer  seems  to  be  sepa- 
rated from  the  cementum  by  a  thin  layer  of  dentin  which  is  clear 
and  apparently  structureless.  This  is  separated  from  the  cementum 
by  a  dark  line,  and  the  first  layer  of  cementum  usually  does  not 
contain  any  lacunae  or  canaliculi.  This  is  supported  by  some 
of  the  experiments  that  have  been  made  with  extracted  teeth.  In 
experimenting  on  the  diffusion  of  drugs  through  dentin,  it  was 
found  that  liquids  sealed  in  the  pulp  chambers  of  extracted  teeth 
could  not  be  detected  in  the  liquids  in  which  the  teeth  were  placed 
unless  the  cementum  was  removed  from  them.  In  the  recent 
experiments  of  Dr.  Southwell,  of  Milwaukee,  in  which  air  was 
forced  through  the  dentin  from  the  pulp  chamber  to  test  the  sealing 
of  cavities  by  filling  materials,  the  air  did  not  escape  from  the 
cementum,  which  would  be  the  case  if  dentinal  tubules  connected 
with  the  canaliculi  of  the  cementum. 

If  the  spaces  of  the  granular  layer  are  filled  with  the  protoplasmic 
enlargements  of  the  ends  of  the  dentinal  fibrils,  this  would  give 
a  very  reasonable  explanation  of  the  sensitiveness  of  slight  caries 
and  erosion  at  the  gingival  line,  as  the  anastomosis  through  the 
granular  layer  would  affect  the  fibrils  of  the  entire  tooth. 

The  Interglobular  Spaces. — There  has  been  considerable  mis- 
understanding in  dental  histology  in  regard  to  these  spaces,  owing 
to  the  confusing  of  two  entirely  different  things.  Tomes  called 
the  spaces  of  the  granular  layer,  \vhich  have  already  been  described, 
interglobular  spaces.  As  has  been  seen,  they  are  true  spaces  in 
the  dentin  matrix  which  connect  with  the  dentinal  tubules  and 
are  filled  with  protoplasm. 

In  1850  J.  Czermak1  described  areas  of  imperfectly  calcified 
dentin  matrix,  which  appear  as  spaces  in  dried  dentin,  and  called 
them  interglobular  spaces.  These  have  been  so  called  by  most 
writers  since.  It  seems  important  to  the  author  that  the  term  be 
restricted  to  these  and  some  other  used  to  indicate  the  spaces 
of  the  granular  layer,  which  are  of  entirely  different  character. 

The  interglobular  spaces  of  Czermak  are  caused  by  some  dis- 
turbance in  the  calcification  of  the  organic  matrix  of  the  dentin 
They  occur  in  zones  (Fig.  125)  which  correspond  to  the  dentin 
matrix,  being  calcified  at  a  given  time,  and  there  is  usually  seen 
a  corresponding  disturbance  in  the  calcification  of  the  enamel, 
which  was  being  formed  at  the  same  time  and  manifested  as  a 
more  or  less  strongly  marked  band  of  Ritzius. 

1  Beitrag  zur  Mikro-Anatoruie  rlf>r  Menschlichen-Zahne. 


THE  INTERGLOBULAR  SPACES 


147 


In  the  calcification  of  the  dentin  matrix  the  inorganic  salts  are 
combined  with  the  organic  matrix  in  spherical  areas  which  become 
united.  The  boundaries  of  these  areas  of  uncalcified  matrix  are 


FIG.  125. — A  drawing  showing  a  zone  of  interglobular  spaces  in  the  dentin. 

(Black.) 


, 


FIG.  126. — Interglobular  spaces  in  dentin.     (About  60  X) 

therefore  very  irregular,  and  made  up  of  concave  facets  where 
they  join  the  spherical  surfaces  of  the  fully  calcified  matrix  (Figs. 
126  and  127).  A  study  of  the  illustrations  and  the  appearance 


148 


THE  DENTIN 


of  the  layer  of  forming  dentin  next  to  the  dental  papilla  of  a  devel- 
oping tooth  will  make  this  intelligible. 

If  the  dentin  is  dried  the  organic  matrix  in  these  areas  gives  up 


! 


J 


FIG.  127. — Interglobular  spaces  in  dentin.    Some  empty,  some  filled  with  debris. 

(About  80  X) 


FIG.    128. — Intci globular  spaces  in  dentin:     Ig,   first  line  of  interglobular    spaces; 
Jij',  second  line  of  interglobular  spaces.     (About  30  X) 


THE  LINES  OF  SCHREGER 


149 


water  and  shrinks,  and  the  interglobular  spaces  become  true 
spaces,  partially  filled  with  the  shrunken  matrix.  In  this  condition 
they  can  be  filled  with  colored  collodion  or  any  other  material 
If,  however,  they  are  studied  in  sections  of  teeth  which  have  never 
been  allowed  to  dry,  no  space  appears,  and  the  dentinal  tubules 
continue  without  change  of  course  or  diameter  through  the  area. 
While  they  are,  therefore,  not  empty  spaces,  they  are  areas  of  the 
organic  basis  of  the  dentin  which  are  bounded  by  globular  surfaces 
of  the  fully  calcified  matrix,  and  their  name  is  properly  significant. 
Zones  of  interglobular  spaces  may  occur  at  any  portion  of  the 
dentin,  either  in  the  crown  or  root,  but  they  are  more  common  in 
the  crown  and  near  the  enamel.  Often  more  than  one  zone  can  be 
seen,  as  in  Fig.  128,  which  shows  two  disturbances  in  calcification, 
and  disturbances  in  the  structure  of  the  enamel  will  be  seen  at 
corresponding  positions. 


FIG.  129. — A  root  broken  on  a  line  of  interglobular  spaces.  This  tooth  was  ex- 
tracted by  Dr.  G.  V.  Black,  and  was  pulled  apart  in  extraction,  a  shows  the  form 
of  the  root  and  a,  b  the  separation  on  the  line  of  growth.  (Black.) 


The  zones  of  interglobular  spaces  appear  in  all  grades,  from  a 
complete  band  of  uncalcified  matrix  to  widely  scattered  patches. 
Fig.  129  shows  a  tooth  in  Dr.  Black's  collection  which  was  broken 
in  extraction,  because  of  the  presence  of  such  a  zone  in  the  root. 

The  interglobular  spaces  are  of  great  importance  in  modifying 
the  direction  of  the  progress  of  caries  in  the  dentin. 

The  Lines  of  Schreger. — As  in  the  case  of  the  interglobular  spaces, 
there  seems  to  be  considerable  misunderstanding  in  the  literature 


150  THE  DENTIN 

and  certain  structures  which  have  very  different  meanings  have 
been  called  the  "lines  of  Schreger." 

An  arrest  in  the  formation  of  dentin  often  occurs  before  the 
crown  is  completed.  When  the  activity  has  begun  again  the 
dentinal  tubules  follow  a  slightly  different  direction.  In  a  longi- 
tudinal section  this  change  in  the  direction  of  the  tubules  produces 
a  line.  Several  such  lines  may  be  seen  in  a  single  section,  though 
they  are  by  no  means  to  be  found  in  all  longitudinal  sections. 

Schreger's  lines  have  been  most  often  confused  with  zones  of 
interglobular  spaces,  and  they  seem  to  be  identical  with  the  incre- 
mental lines  in  the  dentin  described  by  Owen.  It  is  unfortunate 
that  these  names  should  have  been  used,  for  a  thoughtful  study  of 
the  tissue  makes  their  interpretation  perfectly  evident,  and  they 
are  of  no  great  significance. 

Secondary  Dentin. — It  is  by  no  means  easy  to  define  secondary 
dentin  or  to  pick  out  any  particular  piece  of  dentin  in  a  section  and 
to  say  whether  it  is  primary  or  secondary.  In  general,  the  tubules 
are  smaller,  fewer,  and  less  regularly  arranged  in  secondary  than 
in  primary  dentin.  In  general,  it  seems  that  the  smaller  the  remain- 
der of  the  dental  papillae  becomes,  the  more  imperfect  dentin  it 
forms,  until  finally  it  simply  throws  down  granular  calcified  material. 

The  formation  of  dentin  begins  at  the  dento-enamel  junction, 
at  a  number  of  points  in  each  tooth,  and  progresses  from  without 
inward  (strange  to  say,  exactly  the  opposite  statement  has  been 
made  several  times  in  papers  by  very  prominent  men).  This 
matter  will  be  taken  up  more  in  detail  in  the  Chapters  on  Dental 
Embryology  and  Dentition.  It  is  enough  to  say  here  that  in 
studying  all  sections  of  dentin,  whether  cut  longitudinally  or  trans- 
versely, the  formation  of  dentin  began  at  the  dento-enamel  junc- 
tion and  the  dento-cemental  junction,  and  progressed  toward  the 
pulp  chamber. 

From  the  study  of  longitudinal  and  transverse  sections  it  is 
apparent  that  a  certain  typical  amount  of  dentin  is  formed  before 
the  tooth  is  erupted  and  while  it  is  coming  into  full  occlusion. 
This  is  primary  dentin.  In  it  the  tubules  are  very  regular  in  size 
and  arrangement.  From  this  time  on  the  formation  of  dentin  is 
intermittent,  and  apparently  is  the  response  to  some  outside  con- 
dition. These  conditions  may  arise  in  the  tooth  in  which  the  forma- 
tion occurs,  or  the  irritation  of  one  tooth  may  cause  tissue  formation 
in  all  or  part  of  the  others.  It  has  not  been  determined  whether 
such  reflex  trophic  stimuli  are  confined  to  the  same  lateral  half  or 


SECONDARY  DENTIN 


151 


the  same  nerve  distribution.  Apparently  the  formation  of  dentin 
proceeds  again,  after  a  pause,  in  all  teeth.  It  will  seem,  therefore, 
that  the  mere  exposure  of  the  entire  crown  to  conditions  of  thermo- 
change  produces  sufficient  stimulus  to  the  pulp  tissue  to  cause  a 
renewal  of  dentin  formation.  After  the  first  period  of  rest  the 
dentin  formed  in  the  second  period  is  so  nearly  identical,  and  the 
direction  of  the  tubules  so  nearly  the  same,  that  it  is  usually  impos- 
sible to  recognize  the  junction  except  at  a  few  points  in  the  circum- 


FIG.  130. — Secondary  dentin:     A,  margin  of  primary  dentin,  showing  a  few  of  the 
tubules  continuing  into  secondary  dentin;    P,  pulp  chamber.      (About  80  X) 

ference  of  a  transverse  section.  After  each  period  of  rest,  however, 
the  difference  in  structure  between  the  succeeding  portions  becomes 
more  marked.  Fig.  130  shows  an  area  from  a  longitudinal  section 
when  the  line  A  was  the  pulpal  wall  of  the  dentin.  There  was 
probably  a  considerable  period  of  rest,  when  for  some  reason  a 
new  formation  of  dentin  was  begun.  But  apparently  only  some 
of  the  odontoblasts  took  part  in  the  new  formation  of  dentin  matrix, 
for  not  nearly  all  of  the  tubules  are  continued,  and  those  that  do 
continue  show  a  sharp  change  in  their  direction  and  a  difference 
in  diameter  and  character  (Figs.  131  and  132). 


152 


THE  DENTIN 


These  characteristic  changes  in  the  structure  of  the  dentin  that 
is  formed  as  the  pulp  becomes  smaller  seem  to  the  author  of  great 
practical  importance. 


FIG.  131. — A  transverse  section  of  a  root,  showing  the  reduction  in  the  size  of  the 
pulp  and  formation  of  secondary  dentin:  A,  A,  points  at  which  the  changes  in  the 
direction  of  the  tubules  show  dentin  formed  at  different  periods;  C,  cementum 
thickened  and  each  lamella  thicker  in  the  concavity  of  the  dentin;  also,  the  number 
of  lacunae  greater. 


Fiu.   132. — A  transverse  section  of  a  root,  showing  changes  in  the  form  of  the  pulp 
canal  by  the  formation  of  secondary  dentin. 


CHAPTER  XII. 
THE  CEMENTUM. 

THE  cementum  may  be  defined  as  a  connective  tissue  whose 
intercellular  substance  is  calcified  and  arranged  in  layers  (lamellae) 
around  the  circumference  of  a  tooth  root,  the  cells  being  found 
in  spaces  (lacunae)  irregularly  placed  in  or  between  the  layers. 

Structurally  the  cementum  is  more  closely  related  to  the  sub- 
periosteal  bone  than  any  other  tissue,  the  only  differences  being 
that  in  general  the  lacunae  in  bone  are  much  more  uniform  in  size, 
shape,  arrangement  of  the  canaliculi,  and  their  position  \vith  refer- 
ence-to the  lamellae  than  those  in  cementum.  In  bone  the  lacunae  are 
usually  found  between  the  lamellae.  In  cementum  the  lacunae  may 
be  between  the  lamellae,  but  they  are  more  often  enclosed  within 
their  substance  and  they  are  found  most  often  where  the  lamellae 
are  thick. 

Some  writers  have  described  Haversian  canals  in  the  cementum, 
but  the  author  has  never  seen  anything  that  could  properly  be  called 
an  Haversian  canal  in  the  cementum  from  human  teeth.  Canals 
containing  bloodvessels  are  not  uncommon,  but  in  these  the  lamellae 
are  never  arranged  concentrically  around  the  canal,  as  they  are  in 
Haversian  systems.  For  the  last  fifteen  years  the  author  has  had 
under  personal  observation  each  year,  in  the  course  of  class  work, 
not  less  than  200  longitudinal  sections,  and  300  transverse  sec- 
tions of  the  root,  ground  from  human  teeth,  and  in  that  time 
he  has  never  seen  what  could  be  called  an  Haversian  canal.  In 
the  same  time  he  has  examined  many  hundreds  of  sections  cut 
through  the  decalcified  jaws  of  various  mammals,  including  the 
sheep,  pig,  cat,  and  dog,  with  the  same  negative  result. 

Function. — The  function  of  the  cementum  is  to  attach  to  the 
tooth  the  connective-tissue  fibers  which  hold  it  in  position  and 
support  the  surrounding  tissues. 

The  formation  of  cementum  begins  as  soon  as  the  tooth  begins 
to  erupt,  and  continues,  at  least  intermittently,  as  long  as  the 
tooth  remains  in  place,  whether  it  contains  a  live  pulp  or  not. 

The  function  of  the  cementum  cannot  be  too  strongly  empha- 

(153) 


154  THE  CEMENTUM 

sized,  and  must  be  continually  borne  in  mind.  If,  for  any  reason, 
the  tissues  are  detached  from  the  surface  of  the  root,  they  can  only 
be  reattached  by  the  formation  of  a  new  layer  of  cementum  on  the 
surface  of  the  root,  which  will  embed  the  surrounding  connective- 
tissue  fibers.  In  order  to  accomplish  this  the  tissues  must  lie  in 
physiologic  contact  with  the  surface  of  the  root,  and  the  conective- 
tissue  cells  must  be  actively  functional. 

That  the  tissues  may  be  reattached  to  the  surface  of  a  root  is 
both  theoretically  possible  and  clinically  demonstrable,  but  for 
it  to  occur,  biological  laws  must  be  observed  and  the  conditions  are 
very  difficult  to  control,  especially  with  the  old  methods  involving 
the  excessive  use  of  strong  antiseptics.  It  is  well  to  remember 
"that  a  dentist  can  never  cure  a  suppurating  pocket  along  the  side 
of  a  tooth  root,"  but  if  the  conditions  can  be  controlled  the  cells 
of  the  tissue  may  form  a  new  layer  of  cementum,  reattaching  the 
tissues  and  so  close  the  pocket.  It  is  a  biological  problem,  not  a 
matter  of  drugs,  except  as  they  are  a  means  of  producing  cellular 
reaction. 

In  view  of  its  function,  therefore,  the  cementum  becomes  not  the 
least  but  the  most  important  of  the  dental  tissues,  for  no  matter 
how  perfect  the  crown  may  be,  without  firm  attachment  the  tooth 
becomes  useless  and  is  soon  lost. 

Histogenesis.— The  cementum  is  formed  by  connective-tissue 
cells  lying  between  the  fibers  of  the  tissue  which  clothes  the  surface 
of  the  root  and  which  becomes  specialized  for  this  function.  Their 
origin  is  undoubtedly  similar  to  that  of  the  osteoblasts,  but  they 
are  not  osteoblasts,  either  morphologically  or  functionally,  as  will 
be  seen  later  in  the  study  of  the  peridental  membrane,  where  the 
cementoblasts  and  the  formation  of  cementum  will  be  considered. 

Structural  Elements.— The  structural  elements  of  the  cementum 
are: 

1.  The  lamellae. 

2.  The  lacuna?  and  canaliculi. 

3.  The  cement  corpuscles. 

4.  The  ejnbedded  fibers  of  the  peridental  membrane. 

The  Lamellae  of  the  Cementum  and  Their  Arrangement. — The 
lamellae  of  the  cementum  resemble  those  of  bone,  but  they  are  very 
much  more  irregular  both  in  thickness  and  appearance.  They 
may  be  extremely  thin  and  almost  transparent,  or  they  may  be 
quite  thick  and  coarsely  granular.  They  arc  not  nearly  as  easily 
observed  as  those  of  bone,  for  in  bone  the  lamellae  are  marked  off 


THE  LAMELLA  OF  THE  CEMENTUM  155 

by  the  lacunae  which  lie  between  them,  while  in  cementum  the 
lacunae  may  be  entirely  absent,  and  when  present  are  irregularly 
placed. 


••' 

* 

'•'•'-  ^.'- 


cc 


^mmmmmimmm 


FIG.  133. — Hypertrophy  of  the  cementum  on  the  side  of  the  root  of  a  lower  molar 
near  the  neck  of  the  tooth.  From  a  lengthwise  section:  human,  a,  dentin;  6,  cementum; 
c,  fibers  of  peridental  membrane.  From  b  to  c  the  cementum  is  normal  and  the 
incremental  lines  fairly  regular,  but  at  d  one  of  the  lamellae  is  greatly  thickened. 
At  e  this  lamella  is  seen  to  be  about  equal  in  thickness  with  the  others.  The  next 
two  lamellae  are  thin  over  the  greatest  prominence,  but  one  is  much  thickened  at  g, 
and  both  at  h.  These  latter  seem  to  partially  fill  the  valleys  which  were  occasioned 
by  the  first  irregular  growth.  (1  in.  obj.) 

In  the  gingival  portion  of  the  root  the  lamellse  are  always  thin 
and  very  transparent,  and  lacunae  are  seldom  seen.  The  entire 
thickness  of  the  tissue  is  transparent,  and  the  appearance  of  the 


FIG.  134. — Hypertrophy  from  root  of  cuspid:  human,  in  which  the  irregularity  is 
confined  to  the  first  lamella:  o,  dentin;  b,  thickened  first  lamella;  c,  subsequent 
lamella?,  which  are  seen  to  be  fairly  regular.  (1  in.  obj  ) 

lamellae  can  be  seen  only  by  using  a  very  small  diaphragm  or  oblique 
illumination.  In  this  position  the  tissue  is  largely  made  up  of 
embedded  connective-tissue  fibers,  which  are,  however,  so  perfectly 


156 


THE  CEMENTUM 


calcified  that  they  cannot  be  demonstrated  in  ground  sections. 
In  decalcified  sections  they  are  very  easily  seen. 

The  cementum  becomes  gradually  thicker  in  the  middle  third 
of  the  root,  and  is  thickest  in  the  apical  third.  It  will  be  seen  that 
this  increase  in  thickness  is  caused  chiefly  by  the  greater  thickness 
of  each  individual  lamella.  In  longitudinal  sections  the  cementum 


. 

fj^.        />:'V-V    ."'•' ";".••/:    f-r.r'^^h 

r*s&      &f^-r--&'l.'t:&i^a3H2?5&&m  ^t 


FIG.  135.- — -Apex  of  root  of  an  upper  bicuspid  tooth  with  irregularly  developed 
cementum:  a,  a,  dentin;  b,  b,  pulp  canals.  The  lamella?  of  cementum  are  marked 
1,  2,  3,  4,  5,  6,  7,  8,  9;  rf,  d,  d,  absorption  areas  that  have  been  refilled  with  cemen- 
tum. It  will  be  seen  that  the  apices  of  the  roots  were  originally  separate,  but  became 
fused  with  the  deposit  of  the  second  lamella  of  cementum,  and  that  in  this  the  irreg- 
ular growth  began  and  was  most  pronounced.  It  has  continued  through  the  subse- 
quent lamella?,  but  in  less  degree.  It  will  also  be  noticed  that  the  absorption  areas, 
d,  d,  d,  have  proceeded  from  certain  lamellae.  That  between  the  roots  has  broken 
through  the  first  lamella  and  penetrated  the  dentin,  and  has  been  filled  with  the 
deposit  of  a  second  lamella.  Other  of  the  absorptions  have  proceeded  from  lamellae 
which  can  be  readily  made  out.  The  small  points,  e,  seem  to  have  been  filled  with 
the  deposit  of  the  last  layer  of  the  cementum,  while  others  have  one,  two,  or  more 
layers  covering  them.  (2  in.  obj.) 

is  often  found  becoming  suddenly  thicker  at  a  certain  point,  and  if 
examined  closely,  it  will  be  seen  that  each  layer  is  continued  apically, 
but  with  greater  thickness.  Fig.  135  illustrates  this  condition  near 
the  apex  of  the  root.  From  a  study  of  the  lamellte,  therefore,  it 
is  apparent  that  the  entire  root  is  clothed  with  successive  layers, 
and  that  these  layers  are  formed  intermittently,  but  continue  to 
be  formed  as  long  as  the  tooth  is  in  position.  In  a  general  way  the 


THE  LAMELLAE  OF  THE  CEMENTUM  157 

number  of  layers  is  an  index  to  the  age  of  the  person  at  the  time 
the  tooth  was  extracted  (Figs.  136  and  137).  The  rate  of  formation 
is  not  uniform;  for  instance,  a  number  of  layers  may  be  formed 
within  a  short  time,  and  again,  a  considerable  time  may  elapse 
between  the  formation  of  one  layer  and  the  next.  The  time,  however, 
does  not  seem  to  determine  the  thickness  of  the  layer. 

If  a  considerable  number  of  teeth  of  persons  of  twenty  years  of 
age  were  sectioned,  the  lamellae  counted,  and  this  number  compared 
with  the  number  found  in  teeth  extracted  from  persons  of  forty, 


FIG.  136. — A  transverse  section  of  a  root  extracted  from  a  young  person.    The 
cementum  is  thin,  but  is  thicker  in  the  grooves  on  the  proximal  sides. 


a  fairly  regular  increase  in  the  number  of  layers  will  be  noticed,  and 
so  on,  for  fifty,  sixty,  seventy,  or  eighty  years. 

It  is  important  to  remember  in  connection  with  this  formation 
of  cementum  that  the  teeth  move,  more  or  less,  under  the  influence 
of  natural  forces  throughout  life,  and  that  every  slight  change 
in  position  must  be  accomplished  by  the  formation  of  a  new  layer 
of  cementum,  to  reattach  connective-tissue  fibers  in  new  positions 
or  adjust  them  to  new  directions  of  strain. . 

The  first  layer  of  cementum  is  formed  while  the  tooth  is  still  in 
its  crypt,  but  apparently  no  connective-tissue  fibers  are  calcified 
into  it.  This  forms  the  first  apparently  clear  and  structureless 


158 


THE  CEMENTUM 


layer  which  lies  next  to  the  granular  layer  of  Tomes  (Fig.  138). 
Even  in  the  teeth  the  entire  length  of  whose  roots  are  formed 
before  they  begin  to  erupt,  there  is  no  attachment  until  some  stress 
comes  upon  the  crown.  The  tooth  is  lying  loose  in  its  crypt  and  can 
be  picked  out  with  very  little  force.  Bicuspids  are  often  accident- 
ally extracted  in  the  extraction  of  temporary  molars.  As  soon  as 


FIG.  137. — A  transverse  section  of  a  root  from  an  old  person.     This  root  had  carried 
a  crown  for  many  years.     The  section  was  cracked  and  one  edge  broken. 


the  tooth  comes  through  the  gum  a  new  layer  of  cementum  is  formed 
over  the  entire  root,  attaching  the  fibers  to  its  surface,  and  as  the 
tooth  moves  occlusally,  layer  after  layer  is  formed.  This  will  be 
considered  again  in  connection  with  the  peridental  membrane. 

The  Lacunae  and  Canaliculi.- — The  lacunae  of  the  cementum  cor- 
respond with  the  lacunae  of  bone.    They  differ  from  those  of  bone. 


THE  LACUNA  AND  CANALICULI 


159 


however,  in  that  they  are  more  irregular  in  shape,  size,  position, 
and  relation  to  the  lamellae,  and  in  the  number  and  direction  of  the 
canaliculi  radiating  from  them.  In  bone  the  lacunae  are  fairly 
regular  in  shape,  the  long  diameter  exceeding  the  short  diameter 
by  about  one-third.  Sections  cut  through  their  long  axis  give  an 
oval  outline,  the  length  of  which  is  about  three  times  as  great  as 
the  width.  Sections  cut  through  their  short  axis  give  an  oval 
outline,  the  long  diameter  being  about  twice  that  of  the  short. 
The  spaces  are  therefore  flattened  between  the  lamellae.  In 


FIG.  138. — Cementum   near    the   apex  of  the   root:     GT,   granular  layer  of   Tomes; 
L,  lacunae;    B,  point  at  which  fibers  were  cut  off  and  reattached.     (About  54  X) 

cementum  there  is  no  regularity  whatever,  either  in  size  or  in  shape. 
Some  are  a  little  larger  than  the  lacunae  in  bone,  some  are  very  much 
smaller.  They  may  be  almost  exactly  the  shape  of  typical  bone 
lacunae  or  they  may  be  distorted  into  almost  any  form,  sometimes 
being  almost  stellate,  often  pear-shaped,  sometimes  round,  and 
occasionally  pyramidal.  The  lacunae  of  bone  are  fairly  uniformly 
placed,  and  lie  between  one  lamella  and  the  next.1  There  is  no 

This  is  not  absolutely  correct,  there  being  much  more  irregularity  in  the  arrange- 
ment of  the  lacunse  in  thick  subperiosteal  bone  than  in  either  cancellous  or  Haversian 
system  bone.  To  be  strictly  accurate,  the  above  statement  must  be  limited  to 
Haversian  system  bone  (Plate  XIII). 


160  THE  CEMENTUM 

regularity  in  the  relation  of  the  lacunae  of  the  cementum  to  the 
lamellae.  They  sometimes  lie  between  one  lamella  and  the  next, 
but  they  are  more  often  entirely  in  the  substance  of  one.  They 
occur  only  where  the  lamellae  are  thick,  and  there  may  be  large 
areas  with  considerable  aggregate  thickness  of  cementum  in  which 
there  are  no  lacunae  at  all. 

The  number  and  direction  of  the  canaliculi  wrhich  radiate  from 
the  lacunae  of  cementum  is  extremely  irregular,  but  in  general 
there  are  more  extending  from  the  lacunae  toward  the  surface  than 
toward  the  dentin. 

The  Cement  Corpuscles.  —  The  cement  corpuscles  correspond 
exactly  to  bone  corpuscles.  They  are  the  cells  found  in  the  lacunae. 
These  are  simply  embedded  cementoblasts  and  are  typical  connec- 
tive-tissue cells.  They  are  made  up  of  granular  cytoplasm  and 
contain  a  faintly  staining  nucleus.  Extensions  of  the  protoplasm 
undoubtedly  extend  into  the  canaliculi.  These  cells  bear  the  same 
relation  to  the  matrix  of  the  cementum  that  bone  corpuscles  do  to 
that  of  bone.  What  this  is,  is  not  known  in  any  definite  way,  but 
it  is  known  that  when  bone  corpuscles  are  killed  or  die,  the  matrix 
becomes  a  foreign  body,  and  is  either  absorbed  or  cut  off  from  the 
portion  in  which  the  corpuscles  are  living,  to  be  absorbed  or  cast 
out  as  a  sequestrum.  The  same  conditions  are  true  of  cementum. 
For  instance,  there  are  many  cement  corpuscles  in  the  lacunae  in  the 
region  of  the  apex  of  the  root.  If  this  portion  be  bathed  in  pus  for 
a  long  time,  the  cement  corpuscles  are  killed,  and  the  tissue  becomes 
saturated  with  poisonous  materials,  so  that  tissue  cells  cannot  lie 
in  contact  with  it  and  live.  In  order  to  restore  a  healthy  condition, 
the  necrosed  cementum  must  be  removed  mechanically  until  tissue 
is  reached  with  which  cells  may  lie  in  physiological  contact  without 
injury.  Conditions  which  can  only  be  understood  through  a  knowl- 
edge of  the  structure  of  the  tissue  often  arise  in  connection  with  the 
treatment  of  alveolar  abscess.  It  should  always  be  remembered 
that  the  treatment  of  an  abscess  is  a  biological  problem. 

The  Embedded  Fibers  of  the  Peridental  Membrane. — The  embedded 
fibers  of  the  peridental  membrane  are  in  the  strictest  sense  com- 
parable with  the  fibers  of  Sharpe  in  bone.  They  are,  however,  in 
many  places  much  more  perfectly  calcified.  To  appreciate  the 
relation  of  the  embedded  fibers  to  the  matrix,  the  tissue  must  be 
studied  both  in  ground  and  decalcified  sections.  For  instance,  in 
the  gingival  portion,  from  the  study  of  ground  sections,  the  presence 
of  embedded  fibers  would  never  be  suspected,  but  if  decalcified 


EMBEDDED  FIBERS  OF  PERIDENTAL  MEMBRANE      161 

sections  are  studied  it  will  be  found  to  be  almost  entirely  composed 
of  calcified  fibers.  In  the  middle  and  apical  thirds  of  the  root, 
where  the  lamellae  are  thicker,  the  calcification  of  these  fibers  is 
often  not  as  perfect  as  that  of  the  rest  of  the  matrix.  In  the  prepara- 
tion of  ground  sections,  therefore,  the  imperfectly  calcified  fibers 


FIG.  139. — Two  fields  of  cementum  showing  penetrating  fibers:    GT,  granular  layer  of 
Tomes;  C,  cementum  not  showing  fibers;   F,  penetrating  fibers.     (About  54  X) 


shrink  and  consequently  appear  as  canals  in  the  cementum.  In 
fact,  they  have  often  been  mistaken  for  canals.  They  are  usually 
not  seen  unless  the  section  happens  to  cut  in  their  direction.  These 
will  be  seen  in  many  of  the  illustrations  of  cementum.  In  Fig.  138 
several  layers  are  seen  next  to  the  dentin,  in  which  no  fibers  appear, 
11 


162 


THE  CEMENTUM 


then  in  several  layers  the  fibers  are  plainly  seen,  and  finally,  the 
surface  layers  show  no  fibers.  This  probably  means  that  before 
and  after  these  layers  were  formed  there  was  a  change  in  the  position 


D 


FIG.  140. — Record  in  the  calcified  tissue  of  an  absorption  repaired:    D,  dentin; 
Cm,  cementum  filling  absorption  cavity.    (About  40  X) 

of  the  tooth  and  the  fibers  were  all  cut  off  in  this  area  and  reattached 
in  a  new  direction,  adapting  them  to  the  new  directions  of  strain. 


FIG.  141. — Thick  lamellae  of  cementum  with  many  lacuna;,  filling  an  absorption  in 
dentin:    L,  lacunre;    H,  Howship's  lacunce  filled;    D,  dentin.     (About  250  X) 

It  is  often  necessary  to  study  ground  sections  very  closely  to  deter- 
mine whether  certain  appearances  are  embedded  fibers  or  canaliculi 
radiating  from  the  lacuna?.  The  appearance  of  these  fibers  should 


ABSORPTION  AND  REPAIR  OF  CEMENT UM  163 

be  studied  in  Fig.  139.  It  should  be  noted  that  wherever  special 
stress  is  "exerted  upon  a  bundle  of  fibers  the  cementum  is  thick 
around  them.  This  may  be  seen  in  decalcified  sections  in  Figs.  204, 
231  and  Plate  XVII  and  in  ground  sections  in  Figs.  138  and  139. 
When  the  next  layer  is  formed,  if  the  fibers  are  cut  off,  the  additional 
thickness  of  the  last  layer  is  removed.  The  unequal  thickness  of 
the  last  formed  layer  is  not  usually  seen  in  the  layers  beneath  it 
to  as  great  an  extent. 

Absorption  and  Repair  of  the  Cementum. — From  what  has  already 
been  said  about  the  cementum,  it  will  be  understood  that  this  tissue 
is  continually  undergoing  changes,  that  new  layers  are  being  added, 
and  that  often  before  an  addition  is  made  there  is  absorption 
enough  of  it  at  least  to  cut  off  the  fibers.  When  an  absorption 
occurs  on  the  side  of  a  root  which  cuts  into  the  dentin,  the  excava- 
tion in  the  dentin  may  be  filled  by  the  cementum  subsequently 
formed  (Figs.  140  and  141).  From  a  study  of  ground  sections  in 
class  work  such  absorptions  are  not  uncommon.  They  probably 
occur  when  the  cusps  first  come  into  occlusion  in  eruption. 


CHAPTER  XIII. 
DENTAL  PULP. 

Definition. — The  dental  pulp  may  be  defined  as  the  connective 
tissue  occupying  the  central  cavity  of  the  dentin. 

It  is  composed  of  embryonal  connective  tissue  which  is  more 
closely  related  to  the  tissue  occupying  the  spaces  of  cancellous 
bone  than  to  any  other. 

Functions. — The  functions  of  the  dental  pulp  are: 

1.  A  vital  function,  the  formation  of  dentin. 

2.  A  sensory  function  responding  to  thermal  and  chemical  change 
and  traumatic  irritation. 

Vital  Function. — The  vital  function  is  the  formation  of  dentin 
and  is  performed  by  the  layer  of  odontoblasts.  These  cells  also, 
by  means  of  their  dentinal  fibrils,  maintain  the  same  relation  to  the 
dentin  matrix  that  the  bone  and  cement  corpuscles  bear  to  the  matrix 
of  bone  and  cementum.  When  the  pulp  is  removed  from  a  tooth  its 
dentin  becomes  dead  dentin  in  the  same  sense  that  bone  in  which 
the  bone  corpuscles  have  been  killed  is  necrosed  bone.  That  there 
is  a  constant  reaction  between  the  protoplasm  of  the  odontoblasts 
and  the  substance  of  the  dentin  matrix,  or  that  the  presence  of  the 
living  protoplasm  is  necessary  to  prevent  degeneration  of  the  matrix, 
is  evidenced  by  the  changes  in  the  physical  properties  of  the  dentin 
after  the  pulp  has  been  lost.  That  the  tooth  remains  functional 
after  the  loss  of  the  pulp  is  due  to  the  fact  that,  except  at  the  minute 
foramina,  the  dentin  is  not  in  physiologic  contact  with  any  tissue 
excepting  enamel  and  cementum,  and  that  the  cementum  attaches 
the  tooth  to  the  surrounding  tissues,  receiving  its  nourishment 
from  the  surface  and  not  from  the  dentin. 

When  the  pulp  is  removed  and  its  place  filled  by  a  non-irritating 
material,  the  dentin  becomes  entirely  encased  in  cementum,  the 
foramina  probably  being  covered  over  as  the  subsequent  lamellse 
are  formed.  The  author  wishes  to  emphasize,  however,  the  vital 
relations  of  the  pulp  to  the  dentin  matrix.  Dead  dentin  is  never 
as  good  as  living  dentin,  consequently  a  tooth  from  which  the  pulp 
has  been  removed  can  never  be  considered  just  as  good  as  one  with 
the  living  pulp. 
(164) 


VITAL  FUNCTION  165 

The  production  of  the  dentin  matrix  is,  of  course,  the  principal 
part  of  the  vital  function  of  the  pulp.  It  is  begun  in  the  develop- 
ment of  the  tooth  before  the  dental  papilla  is  converted  into  the 
dental  pulp,  by  being  enclosed  in  the  dentin  formed.  After  the 
tooth  is  fully  formed  the  pulp  retains  its  ability  to  build  dentin 
matrix  as  long  as  it  retains  vitality,  but  this  function  is  exercised 
only  in  response  to  conditions  of  environment  which  are  probably 
excited  through  the  intervention  of  its  sensory  function  responding 
to  thermal  change  and  chemical  irritation.  The  sensory  function 
causes  a  trophic  impulse  which  is  manifested  by  the  production  of 
another  portion  of  dentin  matrix  reducing  the  size  of  the  pulp 
chamber.  That  this  is  a.  reflex  and  not  purely  a  local  matter  is 
indicated  by  the  fact  that  formations  of  dentin  occur  in  one  tooth 
when  the  irritation  is  in  another,  and  apparently  the  irritation  of 
one  tooth  will  excite  dentin  formation  in  all  of  the  teeth  on  that 
side,  at  least  in  some  instances.  On  the  other  hand,  purely  local 
responses  are  found  where  a  few  odontoblasts  respond  to  the  irrita- 
tion of  their  fibrils  by  the  formation  of  dentin. 

This  matter  has  been  referred  to  under  the  heading  of  Secondary 
Dentin,  and  it  is  best  studied  by  the  record  it  leaves  in  the  formed 
tissue. 

The  Sensory  Function. — In  regard  to  sensation,  the  pulp  resembles 
an  internal  organ,  as  in  its  normal  condition  it  is  always  enclosed 
in  the  cavity  of  the  dentin.  It  has  no  sense  of  touch  or  localization, 
and  responds  to  stimuli  only  by  sensations  of  pain.  The  pain  is 
usually  located  correctly  with  reference  to  the  median  line,  but 
apart  from  that  it  is  located  only  as  it  is  referred  to  some  known 
lesion.  If  several  pulps  were  exposed  on  the  same  side  of  the  mouth, 
and  in  teeth  of  both  the  upper  and  lower  arches,  so  that  they  could 
be  irritated  without  impressions  reaching  the  peridental  membrane, 
if  the  patient  were  blindfolded  it  would  be  impossible  for  him  to 
tell  which  of  the  pulps  was  touched.  This  characteristic  becomes 
extremely  important  in  diagnosis. 

The  pain  originating  from  a  tooth  pulp  may  be  referred  to  the 
wrong  tooth  or  to  almost  any  point  on  the  same  side  supplied  by 
the  fifth  cranial  nerve. 

The  dental  pulp  is  especially  sensitive  to  changes  in  temperature, 
amounting  almost  to  a  temperature  sense,  having  no  exact  parallel 
in  any  other  tissue  of  the  body.  This  does  not  amount  to  a  recogni- 
tion of  heat  or  cold  as  such,  but  a  special  resentment  to  sudden 
changes.  For  instance,  if  a  tooth  is  isolated  and  so  protected  by 


166  DENTAL  PULP 

non-conductors  that  the  soft  tissues  cannot  be  stimulated,  and  a 
jet  of  hot  and  then  cold  water  be  thrown  upon  its  crown,  it  will 
respond  to  each  with  a  sharp  sensation  of  pain,  but  the  patient 
cannot  tell  which  is  hot  and  which  is  cold.  It  is  the  sudden  change 
that  produces  the  reaction.  This  is  the  basis  of  very  important 
differential  diagnosis,  for,  as  is  true  with  most  organs,  in  pathologic 
conditions  its  sensory  function  is  exaggerated. 

Histogenesis. — The  dental  pulp  is  the  remains  of  the  dental 
papilla;  the  dental  papillae  for  the  temporary  teeth  appearing  in 
the  mesodermal  tissue  of  the  jaw  arches  very  early  in  fetal  life. 
The  cellular  elements  are  at  first  very  closely  placed  and  large, 
but  they  grow  smaller  and  take  on  the  typical  form  of  pulp  cells 
as  the  intercellular  substance  is  increased.  By  the  sixteenth  week 
the  dental  papillae  for  the  temporary  teeth  are  covered  by  a  layer 
of  tall  columnar  cells,  which  will  begin  the  formation  of  dentin  about 
that  time.  After  the  beginning  of  dentin  formation  the  transi- 
tion from  the  dental  papillae  to  the  dental  pulp  is  very  gradual,  and 
it  would  be  impossible  to  draw  any  sharp  line  of  demarcation 
between  them. 

Structural  Elements. — The  structural  elements  of  the  dental 
pulp  are: 

1.  Odontoblasts. 

2.  Connective-tissue  cells. 

3.  Intercellular  substance. 

4.  Bloodvessels. 

5.  Nerves. 

6.  Lymphatic  vessels. 

The  Odontoblasts. — -The  odontoblasts  are  tall  columnar  cells 
which  form  the  outer  layer  of  the  pulp  adjacent  to  the  dentin,  and 
from  which  cytoplasmic  fibrils  extend  into  the  dentinal  tubules. 

The  character  of  the  odontoblasts  changes  very  greatly  with  the 
age  of  the  tissue,  and  the  activity  of  dentin  formation.  While  the 
primary  dentin  is  being  formed  they  are  tall  columnar  cells,  each 
containing  a  large  oval  nucleus,  rich  in  chromatin  and  located  in 
the  pulpal  third  of  the  cell.  From  the  dentinal  end  of  the  cell  cyto- 
plasm is  continued,  without  any  line  of  demarcation,  into  the  den- 
tinal tubule  as  the  dentinal  fibril.  In  some  instances  two  fibrils 
may  be  sent  from  a  single  odontoblast.  The  character  of  the 
odontoblast  is  beautifully  seen  in  Fig.  142,  a  photograph  by  Professor 
Rose. 

After  the  tooth  is  erupted,  but  while  the  formation  of  dentin 
is  actively  going  on,  the  odontoblasts,  while  somewhat  smaller, 


THE  ODONTOBLASTS 


167 


FIG.  142. — Odontoblasts  and  forming  dentin:    E,  forming  enamel;  D,  forming  dentin; 
O,  odontoblasts;    Dp,  body  of  dental  papilla.     (From  photomicrograph  by  Rose.) 


*S  ^^SfffinSrfU 


FIG.  143.— Odontoblasts.  The  section  cuts  obliquely  through  the  odontoblasts; 
F,  fibrils;  A*,  nuclei  of  odontoblasts;  X',  nuclei  of  connective-tissue  cells;  W,  layer 
of  Weil,  not  well  shown.  (About  80  X) 


168 


DENTAL  PULP 


retain  the  same  typical  appearance.  They  may  be  easily  demon- 
strated either  in  decalcified  sections  or  by  removing  pulps  from  the 
pulp  chamber  of  freshly  extracted  teeth.  Professor  Salter  has 
described  two  sets  of  processes  besides  (Fig. 
144)  the  dentinal  fibril  process.  As  a  result  of 
teasing  the  fresh  pulps,  he  considered  that  fine 
projections  of  the  cytoplasm  extended  from 
the  sides  of  the  cells,  uniting  them  to  the  ad- 
joining odontoblasts  (Fig.  144).  These  he 
called  the  lateral  processes.  He  also  described 
cytoplasmic  projections  from  the  pulpal  end  of 
the  odontoblasts  into  the  layer  of  Weil.  It  is 
probable  that  these  appearances  were  the  result 
of  teasing,  and  are  not  true  structural  charac- 
teristics, as  the  work  of  other  investigators  has 
not  confirmed  their  presence.  It  is  easy  to 
understand  how  teasing  the  cells  apart  might 
produce  appearances  which  might  be  interpreted 
as  processes,  but  careful  work  upon  sections 
does  not  show  their  presence. 

In  old  pulps  \vhere  the  formation  of  dentin 
has  been  intermittent  and  very  infrequent  for 
a  long  time,  the  odontoblasts  are  smaller,  lose 
their  columnar  form  more  or  less,  and  become 
pear-shaped  or  globular. 

As  dentin  is  one  of  the  most  highly  specialized 
connective  tissues,  the  odontoblasts  are  among 
the  most  highly  differentiated  connective-tissue 
cells.  They  are  the  only  connective-tissue  cells 
of  columnar  form.  Morphologically,  they  are 
very  similar  to  columnar  epithelium,  but  epi- 
thelial cells  never  have  such  processes  as  the 
dentinal  fibrils.  Occasionally,  in  young  and 
actively  growing  bone,  osteoblasts  are  found 
which  are  distinctly  columnar  in  form,  but  they 
are  never  as  tall  as  the  odontoblasts,  and  the 
nucleus  is  more  nearly  in  the  center  of  the  cell. 
In  the  case  of  the  osteoblast  the  cytoplasmic 

FIG.       144.— Dia-  ,  .  ,  ,    .  ,  ,.      ,. 

gram     of     odonto-     processes  which  extend  into  the  canaliculi  cor- 
blasts  and  dentinal      respond   to  the   dentinai   fibril   process  of  the 

fibrils.     (C.  H.  Sto-  i.ii  rP,         i  i      •  u 

weii)  odontoblast.       Ihe    homologies     between     the 


V 


THE  MEM  BRAN  A  E  BORIS  169 

osteoblasts  and  the  odontoblasts  have  often  been  lost  sight  of 
in  the  discussions  over  the  character  of  the  latter  and  their  relation 
to  the  formation  and  sensitiveness  of  the  dentin. 

The  Membrana  Eboris. — The  odontoblasts  form  a  single  layer  of 
cells  on  the  surface  of  the  pulp  in  contact  with  the  dentin.  This 
layer  was  very  early  recognized  to  be  related  to  the  formation  of 
the  dentin,  and  was  called  the  membrana  eboris,  or  the  membrane 
of  the  ivory.  The  name  has  no  importance  now  except  as  it  is  found 
in  the  literature. 

Size  of  the  Odontoblasts. — From  what  has  been  said  it  will  be 
recognized  that  the  size  and  shape  of  the  odontoblasts  vary  greatly 
in  different  sections.  This  is  true  not  only  of  pulps  from  different 
animals,  and  pulps  at  different  periods  of  development,  but  of 
different  parts  of  the  same  .pulp.  In  the  coronal  portion  of  a  pulp 
from  a  fully  developed  tooth,  but  one  in  which  the  formation  of 
dentin  is  still  going  on,  the  average  measurements  would  be  about 
5ju  in  diameter  and  25  to  30/x  in  height.  During  early  stages  of 
dentin  formation,  before  the  crown  is  fully  formed,  they  are  con- 
siderably larger  and  taller,  and  in  the  pulps  of  a  calf  they  are  much 
larger  than  in  smaller  animals  and  man.  In  a  constricted  pulp, 
as,  for  instance,  in  the  mesial  root  of  a  lower  first  molar,  the  odonto- 
blasts on  the  constricted  sides  will  be  shorter  and  relatively  thicker 
than  on  the  buccal  and  lingual,  where  the  long  axis  of  the  cell  is 
in  the  direction  of  the  long  diameter  of  the  pulp,  but  this  simply 
means  that  the  formation  of  dentin  on  the  constricted  side  is  rela- 
tively farther  advanced  than  on  the  buccal  and  lingual,  and  the 
cells  show  older  phases.  It  is  evident  that  the  supply  of  nourish- 
ment to  the  cells  in  the  constricted  portions  is  more  imperfect, 
and  that  the  ones  farthest  from  the  main  vessels  are  most  affected, 
so  that  dentin  formation  is  slowed  and  made  more  imperfect  here, 
while  it  still  continues  in  full  vigor  around  the  expanded  portions 
of  the  pulp.  This  has  been  spoken  of  in  connection  with  the  study 
of  the  dentin  (see  Figs.  131  and  132). 

Origin  of  the  Odontoblasts. — The  odontoblasts  are  specialized 
connective-tissue  cells.  It  is  therefore  to  be  expected  that  they 
should  be  formed  from  undifferentiated  connective-tissue  cells,  as 
osteoblasts  are  formed  from  similar  cells  of  the  inner  layer  of  the 
periosteum  and  embryonal  cells  of  the  tissue  filling  the  cancellous 
and  marrow  spaces.  The  odontoblasts  are  therefore  developed 
from  embryonal  cells  deeper  in  the  pulp  which  take  their  place 
in  the  odontoblastic  layer.  This  probably  explains  the  appearance 


170  DENTAL  PULP 

of  some  sections,  and  also,  the  author  believes,  the  views  of  some 
men  in  regard  to  the  odontoblasts  and  the  dentinal  fibrils.  In  some 
sections  from  old  pulps  the  odontoblasts  seem  to  be  in  an  incomplete 
layer,  and  their  form  is  more  like  that  of  typical  connective-tissue 
cells. 

In  considering  the  origin  of  the  odontoblasts  it  should  be  noted: 
That  in  the  first  differentiation  of  these  cells  in  the  embryo.  They 
appear  first  where  epidermal  cells  (inner  tunic  of  the  enamel  organ) 
are  in  contact  with  mesodermal  cells  (the  outer  layer  of  the  dental 
papilla).  This  is  true  in  the  formation  of  the  entire  length  of  the 
root — the  enamel  organ — extending  on  down  the  dental  papilla 
beyond  the  point  where  enamel  formation  stops.  (See  Chapter 
XXVI).  The  author  believes  that  the  meaning  and  importance  of 
this  relationship  has  not  yet  been  grasped. 

Connective-tissue  Cells., — The  cells  in  the  dental  pulp,  aside  from 
the  odontoblasts,  are  typical  connective-tissue  cells  such  as  are 
found  in  embryonal  tissue.  They  are  of  three  forms — round, 
spindle-shaped,  and  stellate.  In  the  crown  or  bulbous  portion  the 
cells  are  mostly  stellate,  while  in  the  root  portion  they  are  largely 
spindle-shaped,  with  the  axis  of  the  spindle  parallel  with  the  canal. 
It  seems  difficult  for  students  to  get  an  idea  of  their  arrangement, 
and  the  nucleus  is  often  mistaken  for  the  entire  cell.  The  cells  do 
not  lie  in  contact  in  a  compact  tissue,  but  are  widely  scattered  in 
the  intercellular  substance.  There  is  a  small  ovoid  nucleus,  which 
takes  the  stain  deeply,  surrounded  by  a  mass  of  granular  cytoplasm 
stretching  away  into  very  fine  threads.  In  the  spindle-shaped  cells 
the  cytoplasm  is  stretched  out  in  only  two  directions.  In  the  stellate 
cells  there  may  be  three,  four,  or  more,  stretching  away  in  any  direc- 
tion. Plate  IX  was  very  carefully  drawn  with  the  camera  lucida 
so  as  to  represent  accurately  the  number,  size,  and  position  of  the 
cells  in  that  field  as  seen  with  the  y1^  oil  immersion.  It  is  very 
difficult  in  a  drawing  to  represent  the  third  dimension  of  space, 
and  to  show  that  some  of  the  processes  are  extending  in  a  plane 
at  right  angles  to  the  paper.  An  idea  of  this  can  only  be  obtained 
by  the  very  careful  use  of  the  fine  adjustment  while  studying  the 
cells  with  the  high  power. 

The  round  cells  are  probably  white  blood  corpuscles  or  undiffer- 
entiated  connective-tissue  cells  which  may  develop  either  into  stel- 
late or  spindle-shaped. 

The  Arrangement  of  the  Cells. — Immediately  beneath  the  layer 
of  odontoblasts,  for  a  space  about  one-half  or  two-thirds  as  wide 


PLATE   IX 


A   Field  from  the  Coronal   Portion  of  the  Pulp  from 
a  Human   Molar. 

In  the  corner  the  stage  micrometer  shows  i,1,,,  of  a  millimeter  drawn 
with  the  same  lens.  The  field  shows  the  branching  of  a  bloodvessel  and 
the  connective-tissue  cells  of  the  pulp.  Drawn  from  ,'_.  oil-immersion 
lens  with  caniera  lueida.  (About  12OO  X-) 


THE  INTERCELLULAR  SUBSTANCE  171 

as  the  odontoblastic  layer,  the  cells  are  very  scarce,  making  a  clear 
line  in  many  sections.  This  is  known  as  the  layer  of  Weil,  and 
contains  many  fine  nerve  fibers  which  are  not  stained  by  ordinary 
methods.  Beyond  the  layer  of  Weil  for  a  space  perhaps  twice  as 
wide  as  the  height  of  the  odontoblasts,  the  cells  are  very  closely 
placed.  Through  the  remainder  of  the  pulp  they  are  much  more 
widely  but  comparatively  evenly  scattered. 

The  Intercellular  Substance. — Very  little  is  really  known  about 
the  character  of  the  intercellular  substance  of  the  pulp.  It  contains 
few  fibers,  and  these  in  no  way  resemble  bundles  of  white  or  elastic 
connective  tissue.  The  appearance  in  the  section  is  more  as  if  a 
structureless  gelatinous  material  had  been  coagulated  by  the  reagents. 

There  are,  of  course,  connective-tissue  fibers  in  connection  with 
the  walls  of  the  larger  bloodvessels  and  nerves,  and  to  a  certain 
extent  in  the  gelatinous  material.  In  studying  the  intercellular 
substance  in  the  sections  it  is  necessary  to  remember  that  it  is 
filled  with  the  protoplasmic  projections  from  the  cells,  and  these 
are  stained,  appearing  like  fibers  in  the  matrix.  There  is  need  for 
further  investigation  of  the  character  of  the  intercellular  substance. 

The  Bloodvessels.— The  dental  pulp  is  an  extremely  vascular  tissue, 
and  the  arrangement  of  the  vessels,  the  structure  of  their  walls, 
and  the  nature  of  the  intercellular  substance  through  which  they 
run  render  the  tissue  especially  susceptible  to  the  pathological 
conditions  which  are  associated  with  alterations  in  the  circulation. 

Usually  several  arterial  vessels  enter  the  pulp  through  foramina 
in  the  region  of  the  apex.  These  vessels  have  their  origin  in  the  rich 
vascular  network  of  the  cancellous  bone  (chapter  on  Peridental 
Membrane).  The  arteries  follow  the  central  portion  of  the  pulp, 
giving  off  many  branches  as  they  pass  occlusally,  and  finally  form 
a  very  rich  plexus  of  capillaries  near  the  surface  of  the  pulp.  From 
these  capillaries  the  blood  is  collected  into  the  veins,  which  follow 
courses  parallel  to  the  arteries,  leaving  the  pulp  through  the  same 
foramina  in  the  region  of  the  apex.  It  is  important  to  notice  that 
an  artery  is  entering  and  a  vein  leaving  the  tissue  through  very 
minute  canals  in  the  calcified  dentin  (Fig.  145).  Dr.  Stowell  has 
made  a  very  beautiful  diagram  of  the  arrangement  of  the  blood- 
vessels in  a  single-rooted  tooth,  which  is  shown  in  Plate  X.  Prep- 
arations such  as  would  reproduce  this  diagram  can  be  made  by 
injecting  the  bloodvessels  with  an  inert  material  and  destroying 
the  soft  tissues  by  artificial  digestion. 


172 


DENTAL  PULP 


Toward  the  periphery  of  the  pulp  very  delicate  vessels  pass 
outward  terminating  in  loops  just  beneath  the  odontoblasts. 
These  are  shown  in  Fig.  146. 


FIG.  145. — A  section  through  the  apex  of  a  root  showing  three  foraminse,  A,  B, 
and  C.    (Talbot.) 

Structure. — The  delicacy  of  the  walls  of  the  bloodvessels  is  one 
of  the  most  striking  histologic  characteristics  of  the  dental  pulp. 


PLATE  X 


Bloodvessels  of  the  Dental  Pulp.     (After  Stowell  ) 

A  well-injected  pulp  studied  under  a  binocular  microscope  make^  a 
very  beautiful  object  which  no  Hat  picture  can  represent.  The  larger 
bloodvessels  lying  at  the  centre  branch  and  divide,  forming  a  network 
which  becomes  very  fine  at  the  surface. 


THE  BLOODVESSELS 


173 


The  largest  arteries  show  only  a  few  muscle  fibers  in  the  media 
and  a  very  slight  condensation  of  fibrous  tissue  for  an  adventitia. 
There  is  no  distinct  boundary  between  the  capillaries  and  the  veins, 
and  the  vessels  continue  to  have  only  a  wall  of  endothelial  cells 
after  they  have  reached  a  size  much  greater  than  that  of  capillaries. 
Because  of  this  peculiarity  of  structure  the  statement  is  to  be  found 
in  many  text-books  of  histology  that  the  largest  capillaries  in  the 
body  are  found  in  the  dental  pulp.  These  vessels  should  probably 
not  be  considered  as  capillaries,  but  as  veins  whose  walls  have  the 
structure  of  capillaries.  Even  in  the  largest  veins  the  media  is  very 


FIG.  146. — Dental  pulp  showing  bloodvessel  loops  extending  to  the  periphery,  close 
to  the  layer  of  odontoblasts. 

imperfect,  and  there  is  only  a  slight  condensation  of  fibrous  tissue 
to  represent  the  adventitia.  This  peculiarity  of  the  bloodvessel 
walls  in  the  pulp  renders  the  tissue  peculiarly  susceptible  to  hyper- 
emia  and  inflammation. 

Fig.  147  is  a  photograph  of  a  bloodvessel  whose  size  can  be 
estimated  from  the  number  of  red  corpuscles  seen  in  it,  and  the 
wall  is  made  up  of  a  single  layer  of  endothelial  cells.  There  is  no 
indication  of  either  media  or  adventitia.  The  intercellular  substance 
of  the  pulp  being  of  gelatinous,  semifluid  character,  gives  no  support 
to  these  delicate  walls, 


174 


DENTAL  PULP 


In  Plate  IX  the  author  has  drawn  very  carefully,  with  the 
camera  lucida,  using  a  yV  immersion  lens,  a  field  showing  the  branch- 
ing of  a  small  bloodvessel.  The  size  of  the  endothelial  cells,  position 
of  their  nuclei  in  the  wall  of  the  vessel,  and  the  size,  position,  and 
shape  of  the  connective-tissue  cells,  are  represented  as  accurately 
as  possible.  The  field  is  from  the  coronal  portion  of  the  pulp  of  a 
human  molar.  The  caliber  of  such  a  vessel  as  this  would  depend 


FIG.  147. — A  pulp  bloodvessel,  showing  the  thin  wall:  C,  blood  corpuscles  in  the 
vessel;  Bl,  bloodvessel  wall  showing  nuclei  of  endothelial  cells;  N,  nuclei  of  con- 
nective-tissue cells  in  the  body  of  the  pulp;  /,  intercellular  substance,  showing  a  few 
fibers.  (About  200  X) 

almost  entirely  upon  the  blood-pressure.  The  endothelial  cells 
will  stretch  to  a  very  considerable  extent  under  increased  pressure, 
becoming  very  thin  at  all  points  except  around  the  nucleus.  When 
the  pressure  is  decreased  the  contractility  of  the  cytoplasm  pulls 
the  cells  together,  making  it  thicker  and  less  in  diameter.  It  is 
very  important  to  remember  these  facts  in  connection  with  hyper- 
emia  of  the  dental  pulp.  It  is  difficult  in  such  an  illustration 
to  give  any  representation  of  the  third  dimension  of  space,  which 


PLATE  XI 


A  Field  from  the  Pulp  of  an  Unerupted  Tooth  of  a  Sheep. 

The  bloodvessels  are  cut  transversely.     'About  1OOOX.) 


LYMPHATICS  OF  THE  DENTAL  PULP  175 

is  essential  to  a  real  understanding  of  the  connective-tissue  cells 
of  the  pulp.  These  are  bits  of  cytoplasm  with  a  nucleus  forming 
a  small,  irregular  central  mass,  from  which  the  cytoplasm  is  stretched 
away  in  all  directions  through  the  intercellular  substance,  ending 
in  very  fine  threads. 

Plate  XI  is  drawn  in  the  same  way  from  a  transverse  section  of 
the  pulp  of  an  unerupted  tooth  of  a  sheep.  The  vessels  are  all 
cut  transversely  and  are  seen  crowded  with  red  blood  corpuscles. 
They  are  not  distended,  and  some  show  slight  condensation  of  fibrous 
tissue  around  them. 

In  a  normal  pulp  there  are  many  capillaries  so  small  that  a  single 
corpuscle  passes  them  with  difficulty,  but  in  pathologic  conditions 
they  become  distended  to  many  times  their  normal  diameter. 

Lymphatics  of  the  Dental  Pulp. — It  was  for  a  long  time  believed 
that  the  dental  pulp  contained  no  lymphatic  vessels.  In  1907-1909, 
Schweitzer  succeeded  in  injecting  lymphatic  vessels  in  the  pulp.1 
In  1916-1917  Dr.  K.  Dewey  and  the  author  repeated  the  work  of 
Schweitzer  in  the  Histological  Laboratory  of  the  College  of  Dentistry 
University  of  Illinois,  and  also  succeeded  in  injecting  the  lymphatic 
glands  of  the  neck  in  dogs  by  injections  in  the  dental  pulp.2 


FIG.  148 

Fig.  148  shows  a  portion  of  the  pulp  of  a  young  dog.  The  blood- 
vessels are  injected  with  gelatin  carmin,  the  lymphatics  with  Berlin 
blue.  Very  fine  vessels  were  found  close  to  the  surface  of  the 
dentin  (Fig.  149).  From  these  capillaries  vessels  pass  through  the 

1  Schweitzer:     Ueler   die  lymphgefasse   des   Zahnfleisches  und    der   Zahne    beim 
Menschen  und  bei  Saiigethieren,  Archiv.  f.  Micr.  Anat.,  1907,  p.  807,  1909,  p.  27. 

2  A  Study  of  the  Lymphatic  Vessels  of  the  Dental  Pulp,  Dental  Cosmos,  vol.  lix, 
1917,  pp.  436-44;  Journal  of  the  American  Medical  Association.     Oct.  12-1918,  vol. 

ii,  pp    1179-1184.  ' 


176 


DENTAL  PULP 


central  portions  of  the  tissue  and  pass  through  the  apical  foramina 
where  they  anastomose  with  the  vessels  of  the  peridental  mem- 


FIG.  149. — Diagrammatic  drawing  of  a  section  of  a  tooth,  showing  injected 
lymphatic  vessels  in  the  pulp. 


FIG.  150 


brane  (Fig.  164).  For  their  course  from  this  point  see  p.  195.  In 
the  body  of  the  pulp  independent  lymph  vessels  are  found  and  peri- 
vascular  lymph  sheath  surrounding  bloodvessels  (Fig.  150). 


THE  NERVES  OF  THE  DENTAL  PULP  177 

The  Nerves  of  the  Dental  Pulp. — Few  subjects  in  connection  with 
dental  histology  have  received  more  attention  than  the  distribu- 
tion of  the  nerves  of  the  dental  pulp,  especially  in  relation  to  the 
sensitiveness  of  the  dentin.1 

For  fifteen  years  or  more  Dr.  Howard  Mummery  has  been  doing 
work  on  the  distribution  of  the  nerves  of  the  dental  pulps.  He  has 
described  nerve-end-cells  lying  between  the  odontoblasts  at  their 
pulpal  end,  from  which  neuro-fibrils  extend  through  the  layer  of 
odontoblasts  and  enter  the  dentinal  tubules  with  the  fibers  of  Tomes. 
According  to  his  description  these  cells  form  true  sensory  neurons  the 
axon  of  which  extend  throughout  the  dentin  in  the  dentinal  tubules, 
their  dendrons  connecting  with  the  terminal  fibrils  of  the  axons 
entering  the  pulp  through  the  apical  foramina  He  considers  the 
odontoblasts  as  the  builders  of  the  dentin  matrix,  or  at  least  the 
calcification  of  it,  and  the  nerve-end-cells  to  perform  the  sensory 
functions  formerly  ascribed  to  the  odontoblasts  and  their  fibrils. 

Support  for  almost  any  idea  can  be  found  in  the  literature, 
but  many  of  the  conditions  described  have  been  shown  to  be 
errors  in  microscopic  interpretation,  and  many  others  have  failed 
to  receive  support  by  reinvestigation.  The  most  recent  work  in  this 
country  upon  this  subject  was  done  fifteen  or  twenty  years  ago 
by  Prof.  Carl  Huber,  of  Ann  Arbor.  The  author  has  repeated 
some  of  his  work,  and  has  never  seen  any  specimen  that  was  con- 
tradictory to  his  statements.  Usually  three  or  four  nerve  trunks 
enter  the  dental  pulp  through  the  foramina.  .These  contain  from 
eight  or  ten  to  thirty  or  forty  medullated  nerve  fibers.  They  pass 
occlusally  through  the  central  portion  of  the  pulp,  but  almost 
immediately  begin  to  give  off  branches,  which  pass  toward  the 
periphery,  branching  and  anastomosing  in  their  course.  Most  of 
the  fibers  lose  their  medullary  sheath  very  soon  after  leaving  the 
nerve  trunk,  proceeding  as  beaded  fibers,  made  up  of  an  axis 
cylinder  with  nuclei  scattered  along  it.  A  bundle  of  such  fibers, 
breaking  up  to  be  distributed  to  one  horn  of  the  pulp,  is  shown  in 
Fig.  151.  Other  fibers  retain  their  medullary  sheath,  following  an 
independent  course  through  the  pulp  tissue,  until  they  reach  the 
layer  of  Weil,  where  the  sheath  is  lost  and  they  join  the  plexus 
of  beaded  fibers  lying  in  this  position  (Fig.  151).  From  the  plexus 

1  Several  investigators  have  described  nerve  fibers  entering  the  dentinal  tubules. 
The  most  complete  and  elaborate  work  is  that  of  Howard  Mummery.  For  which  the 
student  is  referred  to,  Microscopic  Anatomy  of  the  Teeth.  J.  Howard  Mummery, 
p.  211. 

12 


178  DENTAL  PULP 

in  the  layer  of  Weil  beaded  fibers  are  given  off,  passing  between 
and  around  the  odontoblasts,  forming  a  network  around  each  cell, 
and  even  passing  over  on  to  the  end  of  the  cell  between  it  and  the 
dentin,  but  they  have  never  been  followed  into  the  dentinal  tubules. 
In  no  instance  and  by  no  method  that  he  has  employed,  has 
Dr.  Huber  been  able  to  demonstrate  nerve  fibers  in  the  dentinal 
tubules. 

The  sensitiveness  of  the  dentin,  in  view  of  these  observations, 
is  due  to  the  presence  of  living  fibrils,  connected  with  living  odonto- 
blasts which  are  in  physiologic  connection  with  nerve  fibers.  It 


FIG.  151. — -Nerve  fibers  in  pulp  from  a  human  molar.     (About  500  X) 

is  interesting  to  note  that  this  is  the  only  instance  in  which  a  con- 
nective-tissue cell  is  intermediate  between  the  outside  world  and 
the  nerve  fiber.  In  all  other  instances  an  epithelial  cell  is  inter- 
mediate between  the  environment  and  the  nervous  system.  The 
sensitiveness  of  the  dentin  is  therefore  due  to  the  irritability  of  the 
cytoplasm  of  the  fibril,  transmitted  through  the  continuity  of  cyto- 
plasm to  the  odontoblasts  and  their  reaction  upon  the  surrounding 
nerve  fibers.  The  irritation  to  the  fibril  may  be  either  traumatic, 
chemical,  or  thermal.  For  instance,  salt  is  sprinkled  on  exposed 
living  dentin,  and  a  sharp  sensation  of  pain  is  the  result.  It  may  be 
supposed  that  chemical  changes  are  set  up  in  the  cytoplasm  of  the 
fibril  which  excite  changes  in  the  cytoplasm  of  the  odontoblasts. 


THE  NERVES  OF  THE  DENTAL  PULP 


179 


These  react  upon  the  cytoplasm  of  the  nerve  fiber,  and  so  are  trans- 
mitted to  the  nerve  center,  being  recognized,  in  consciousness,  as  a 
sensation  of  pain.  In  the  same  way  traumatic  irritation  caused, 
for  instance,  by  the  cutting  of  dentin  with  a  steel  instrument  sets 
up  changes  in  the  fibril  in  the  same  fashion.  It  is  impossible  to 
conceive  of  any  vital  activity  of  cytoplasm  otherwise  than  as  a  form 
of  chemical  action  or  molecular  or  atomic  movement  of  its  substance. 
Certain  clinical  facts  are  well  explained  by  these  structural 
facts.  It  is  often  noted  in  the  preparation  of  cavities  that  the 
dentin  is  most  sensitive  at  the  dento-enamel  junction.  This  would 
be  expected  when  it  is  recalled  that  at  the  dento-enamel  junction 
the  dentinal  tubules  fork  and  the  fibrils  anastomose,  so  that  an 


FIG.  152. — Rose's  diagram  of  nerves  and  bloodvessels  of  the  pulp. 

irritation  to  a  few  fibrils  is  not  simply  transmitted  to  their  odonto- 
blasts  and  the  nerve  endings  in  contact  with  them,  but  to  all  the 
fibrils,  and  so  to  the  nerves  in  contact  with  all  of  the  odontoblasts. 
The  presence  of  dilute  acids  render  the  cytoplasm  of  the  fibrils 
much  more  irritable.  The  dentin  in  a  carious  condition  is  therefore 
much  more  sensitive  than  that  in  a  sound  or  normal  area.  The 
sensitiveness  of  extremely  hypersensitive  dentin  can  often  be 
greatly  reduced,  if  not  entirely  overcome,  by  cleansing  the  cavity 
thoroughly,  washing  with  tepid  water,  followed  by  a  dilute  alkali, 
drying  and  sealing  for  a  few  days,  when  it  will  be  usually  found 
that  excavation  can  be  carried  out  without  excessive  pain.  The 
sealing  must  be  perfect.  If  it  is  leaky  the  cavity  will  be  more  sensi- 
tive than  ever  at  the  end  of  the  delay. 


180  DENTAL  PULP 

Teeth  in  which  the  size  of  the  pulp  chamber  has  been  reduced 
by  the  formation  of  secondary  dentin  are  usually  much  less  sensitive. 
By  this  formation,  as  has  been  seen  in  the  chapter  on  dentin,  many 
of  the  tubules  are  cut  off  and  many  of  the  fibrils  reach  the  pulp 
only  by  anastomosing  with  a  few  in  the  later  formed  dentin.  The 
transmission  to  the  nerves  of  the  pulp  is  thus  made  more  difficult 
and  imperfect. 

In  all  considerations  of  the  sensitiveness  of  dentin,  the  purely 
subjective  and  hysterical  symptoms  must  be  carefully  watched  for. 
In  many  cases  slight  sensations  are  so  magnified  by  fear  and  expec- 
tation as  to  be  considered  intolerable.  In  such  cases  the  diversion 
of  attention  and  the  skilful  use  of  suggestions  are  of  more  value 
when  coupled  with  delicacy  of  manipulation  and  operative  skill 
than  any  means  of  obtunding.  In  such  cases,  although  the  operator 
is  positive  that  the  sensations  are  slight,  it  will  never  do  any  good 
to  tell  the  patient  so,  or  to  argue  that  what  is  being  done  cannot 
hurt.  They  must  be  made  to  believe  fully  that  something  has  been 
done  to  destroy  the  sensitiveness,  and  then  the  attention  must  be 
concentrated  upon  something,  while  the  excavation  is  lightly  and 
skilfully  performed.  It  makes  very  little  difference  what  is  done, 
but  it  must  attract  the  attention  in  order  to  plant  the  belief  that  the 
sensitiveness  has  been  removed,  and  then  the  attention  must  be 
diverted  until  the  manipulation  is  completed. 

The  nerves  of  the  pulp  not  only  respond  with  sensations  of  pain 
from  the  irritation  of  the  fibrils  in  the  dentinal  tubules,  but  because 
of  their  confinement  in  a  calcified  chamber  and  the  semifluid  nature 
of  the  tissue,  they  are  very  sensitive  to  pressure,  either  increased 
or  decreased.  The  normal  response  to  changes  of  temperature,  as 
well  as  most  of  the  pain  in  pathologic  conditions  of  the  pulp, 
are  probably  caused  by  changes  of  pressure,  through  disturbance 
of  the  blood  circulation  of  the  tissue.  The  nerves  of  the  pulp  con- 
trol the  walls  of  the  arteries  through  the  vasomotor  reflexes,  and 
also  by  trophic  fibers  control  the  functional  activity  of  the  odonto- 
blasts  in  the  formation  of  the  dentin. 

In  a  single  tooth  the  irritation  resulting  from  a  carious  cavity 
is  found  to  cause  the  formation  of  dentin  not  simply  in  the  region 
reached  by  the  irritated  fibrils,  but  upon  the  entire  wall  of  the  pulp 
chamber  and  apparently  also  in  other  teeth.  It  has  seemed  possible 
to  the  author  that  in  some  instances  osmotic  conditions  might  be  a 
factor  in  the  production  of  pain  in  the  pulp,  especially  in  the  early 
stages  of  caries. 


CHAPTER  XIV. 
THE  LYMPHATICS  OF  THE  DENTAL  REGION. 

GENERAL  CONCEPTION   OF  THE  LYMPHATIC  CIRCULATION. 

THE  student  generally  finds  difficulty  in  getting  any  clear  idea  of 
the  lymphatic  circulation.  It  seems  best,  therefore,  to  make  a 
most  simple  and  elementary  statement  of  this  most  important 
circulatory  system  as  a  basis  for  a  study  of  the  lymphatic  vessels. 
Life  at  present  can  be  understood  only  in  terms  of  a  single  cell. 
Every  living  cell  must  be  bathed  in  fluid  from  which  the  cytoplasm 
receives  the  material  for  its  constructive  processes  and  to  which  it 
gives  up  its  waste  products  or  the  results  of  catabolism.  Just  as 
the  single-celled  protozoan  floating  in  a  pond  of  water,  so  each  cell 
of  every  tissue  of  the  body  can  be  considered  as  bathed  in  a  fluid — 
the  lymph.  The  epithelium  of  all  external  and  internal  surfaces 
makes  a  bounding  layer  which  prevents  the  loss  of  the  fluid.  If  a 
slight  cut  or  abrasion  is  made  on  the  skin,  removing  the  outer  layer 
of  dried  cells  and  not  breaking  the  blood  capillaries,  there  will  appear 
the  exudation  of  a  drop  of  yellowish  fluid  on  the  surface.  This 
fluid  immediately  coagulates  and  prevents  further  loss  until  the 
continuity  of  the  surface  is  restored.  In  this  simple  way  we  may 
demonstrate  the  presence  of  the  intercellular  fluid  or  lymph. 

For  the  health  and  nourishment  of  the  cells  this  fluid  must  be  in 
circulation  or  the  cells  would  be  poisoned  by  their  own  products  of 
catabolism.  In  a  very  general  way  the  blood  circulatory  system 
may  be  said  to  be  the  means  of  bringing  oxygen  to  the  tissues  and 
the  lymph  circulatory  system  the  means  of  supplying  the  material 
for  metabolism. 

The  fluid  of  the  blood  passes  through  the  cells  of  the  capillary 
walls  into  the  intercellular  and  tissue  spaces,  and  in  that  sense  may 
be  considered  the  source  of  the  lymph.  The  passage  of  the  blood 
plasma  through  the  capillary  walls  is  not  simply  a  matter  of  trans- 
fusion or  osmosis,  but  is  a  vital  function  of  the  cells  of  the  capillary 
walls.  The  intercellular  lymph  is  not  the  same  as  the  plasma  of  the 
blood  in  the  bloodvessels,  for  from  it  the  cytoplasm  of  the  tissue  cells 
have  taken  up  material  and  to  it  they  have  given  products  of 
metabolism. 

(181) 


182  LYMPHATICS  OF  THE  DENTAL  REGION 

The  fluid  from  the  intercellular  and  tissue  spaces  is  collected  by  a 
system  of  vessels,  the  lymphatic  vessels,  and  returned  to  the  blood 
circulation  through  the  thoracic  duct  emptying  into  the  left  sub- 
clavian  vein.  On  the  right  a  very  short,  lymphatic  duct,  not  more 
than  10  to  12  mm.  in  length,  empties  into  the  right  subclavian  vein. 
Very  frequently  no  right  lymphatic  duct  exists,  the  jugular  and  sub- 
clavian trunks  opening  independently  into  the  right  subclavian  vein. 

Formerly  it  was  supposed  that  the  smallest  of  the  lymph  vessels 
or  lymph  capillaries  opened  directly  into  the  intercellular  and  tissue 
spaces,  but  it  has  become  more  and  more  evident  that  this  is  not 
correct  but  that  the  lymphatic  vessels  form  a  closed  system  opening 
only  into  the  subclavian  veins.  The  intercellular  fluid  passes  into 
the  lymph  capillaries  through  their  wall  by  a  vital  process.  A 
diagram  of  the  lymphatic  vessels  and  their  relation  to  the  blood 
circulation  is  shown  in  Plate  XII. 

It  is  undoubtedly  true  that  the  blood  capillaries  also  may  take  up 
fluid  from  the  tissue  as  well  as  give  up  fluid  to  it  and  it  is  certain  that 
they  take  up  products  of  metabolism  from  the  tissue  cells.  But  as 
a  beginning  and  elementary  idea  the  statement  may  be  made  that 
the  plasma  of  the  blood  passes  out  of  the  capillaries,  bathes  the  cells, 
giving  up  material  to  them  and  receiving  products  from  them, 
and  is  returned  to  the  blood  circulation  through  the  lymphatic 
vessels. 

In  comparing  the  two  systems  in  Plate  XII  several  things  can  be 
noted:  (1)  The  blood  passes  from  the  heart,  through  the  arteries 
to  the  capillaries  and  back  to  the  heart  in  the  veins;  and  is  a  closed 
system  all  the  way.  The  lymph  is  collected  from  the  tissue  spaces 
by  the  lymphatic  capillaries,  passes  through  collecting  trunks  to  the 
glands,  where  it  passes  through  the  capillaries  again  and  on  to  the 
blood  circulation  through  the  subclavian  vein.  (2)  The  blood  cir- 
culation is  the  oxygen  carrier,  the  lymphatic  circulation  the  food 
and  waste  carrier.  (3)  The  blood  circulation  is  rapid,  the  lymph 
circulation  slow. 

Lymphatic  Nodes  or  Glands. — Along  the  course  of  the  lymphatic 
vessels  are  placed  structures,  lymphatic  nodes  or  glands  in  which 
the  fluid  must  come  in  contact  with  masses  of  active  cells  for  the 
purpose  of  preventing  infection  carried  in  the  current  from  reaching 
the  blood  circulation  and  so  the  entire  body.  For  the  structure  of 
the  lymph  nodes  and  their  relation  to  the  lymphatic  vessels  the 
student  is  referred  to  text-books  of  histology  and  anatomy. 


General  Scheme  of  the  Lymphatic  System 


PARTS  OF  THE  LYMPHATIC  SYSTEM  183 

PARTS  OF  THE  LYMPHATIC  SYSTEM. 

To  have  a  conception  of  this  system,  the  fluid  that  circulates,  the 
cells  it  carries,  the  vessels  through  which  it  goes,  and  the  tissue  or 
special  structures  through  which  it  passes  in  its  course,  must  be 
studied  in  their  relation  to  each  other. 

1.  Lymph. 

2.  Leukocytes  (cells  found  in  the  lymph). 

3.  Lymph  vessels. 

4.  Lymphatic  glands  (lymph  nodes). 

Lymph. — The  lymph  is  a  slightly  viscous  liquid,  sometimes  with 
slightly  yellowish  color,  no  or  very  slight  odor,  slightly  alkaline 
reaction,  and  specific  gravity  of  1.012  to  1.022.  Krause  states  that 
the  entire  quantity  of  lymph  is  equal  to  one-third  of  the  body  weight. 
Five  and  one-half  liters  have  been  collected  from  the  thoracis  duct 
from  man  in  twenty-four  hours.  The  quantity  is  dependent  upon 
tissue  activity. 

From  the  most  fundamental  conception  of  it  the  lymph  must  be 
slightly  different  from  the  plasma  of  the  blood.  And  its  chemical 
composition  must  be  variable.  It  is  slightly  less  alkaline  and 
contains  less  fibrin  than  the  blood  plasma. 

Leukocytes. — The  term  leukocytes  includes  cells  that  are  found 
in  the  blood,  lymph,  and  connective  tissues,  and  is  synonymous 
with  white  blood  corpuscles. 

The  leukocytes  are  soft  cytoplasmic  masses  with  no  cell  wall, 
nearly  colorless,  extensible,  and  of  varying  refraction.  They  are 
heavier  than  lymph  or  plasma  and  lighter  than  red  corpuscles.  They 
are  viscous,  adhering  to  a  glass  slide  and  sticking  to  the  walls  of 
vessels,  resisting  the  current  which  carries  them  along,  so  that  they 
accumulate  when  the  current  slackens. 

They  possess  all  the  biological  properties  of  primitive  cells, 
mobility,  sensibility,  absorption,  secretion  and  reproduction. 
Such  important  functions  as  the  absorption  of  foreign  matter  and 
bacteria  are  dependent  upon  these  primitive  functions. 

Leukocytes  have  been  classified  by  their  form,  size,  the  character 
of  the  nucleus  and  the  granules  found  in  the  cytoplasm. 

Lymphatic  Vessels. — Lymphatic  vessels  were  discovered  by  the 
ancient  Greeks  and  were  known  by  Aristotle  (384-322  B.C.),  but 
the  knowledge  was  lost  and  they  were  rediscovered  by  Nicholas 
Massa  in  1532  A.D.  In  1563  Eustachius  discovered  the  thoracic 
duct. 


184 


It  was  formerly  believed  that  the  lymph  in  the  intercellular 
spaces  drained  into  the  interfibrous  spaces  in  the  connective  tissues, 
that  these  became  lined  with  endothelial  cells  and  that  the  lymph 
capillaries  opened  into  them.  It  has  been  more  and  more  apparent 
that  the  lymphatic  vessels  present  a  system  closed  at  the  periphery, 
and  opening  into  the  subclavian  vein  at  the  opposite  extremity 
This  does  not  in  any  way  change  the  action  of  the  system.  The 

taking  up  of  the  lymph  from  the 
tissue  spaces  cannot  be  thought  of 
as  a  simple  process  of  filtration  but 
as  a  vital  function  of  the  cells  form- 
ing the  closed  ends  of  this  term- 
inal or  collecting  plexus  of  the  lym- 
phatic capillaries.  The  entire 
system  of  the  lymph  vessels  may 
be  more  clearly  understood  if  it 
is  thought  of  as  made  up  of  the 


If- 


FIG.  153  FIG.  154 

FIGS.  153  and   154. — Lymphatics  in  involution.     Fig.    153,  lymphatic  vesicle 
continuity  with  neighboring  trunk;  Fig.  154,  isolated  vesicle.     (After  Ranvier.) 

following  parts:  (1)  The  network  of  origin  or  terminal  plexus 
of  the  lymphatic  capillaries  which  take  up  the  lymph  from  the 
tissues  and  organs.  (2)  A  few  vessels  collecting  trunks  drain  a  com- 
paratively large  area  of  the  collecting  capillary  network  and  carry 
the  lymph  from  the  network  to  the  first  lymphatic  gland.  (3)  In 
the  gland  or  node  it  again  breaks  up  into  capillaries,  but  leaves  the 
gland  through  one  vessel,  the  efferent  vessel.  (4)  Larger  and  less 
numerous  efferent  ducts  which  carry  the  lymph  from  one  node  to 
another  or  from  the  last  node  to  the  venous  system. 


PARTS  OF  THE  LYMPHATIC  SYSTEM  185 

The  structure  of  the  vessels  is  different  in  the  different  parts  but 
may  be  described  in  general  by  saying  that  the  capillaries  and  small 
collecting  vessels  are  lined  by  a  single  layer  of  exceedingly  delicate 
endothelial  cells  and  the  larger  trunks  show  three  layers  similar 
to  the  walls  of  the  veins  but  more  delicate  in  structure  (Figs.  153 
and  154). 


Fia.  155 

As  a  general  statement  the  network  of  origin  is  in  the  subepithe- 
lial  connective  tissue.  The  collecting  and  transporting  trunks  are 
found  in  the  connective  tissue  and  are  either  superficial  or  deep,  as 
they  are  above  or  below  the  fascia.  The  superficial  vessels  are 
usually  more  highly  developed. 

The  total  capacity  of  the  network  of  origin  is  very  great,  being 
equal  to  or  greater  than  that  of  the  veins,  but  the  capacity  is  greatly 
reduced  in  the  collecting  and  efferent  ducts,  so  that  the  entire  system 


186 

is  representative  of  a  cone,  with  the  base  in  the  network  of  origin  and 
the  apex  in  the  opening  into  the  subclavian  veins. 

There  are  two  entirely  independent  systems  of  the  lymphatic 
vessels,  one  emptying  into  the  right  subclavian  vein  through  the 
right  lymphatic  duct,  draining  the  right  side  only  as  far  as  the  level 
of  the  diaphragm,  and  the  other  into  the  left  subclavian  vein  through 
the  thoracic  duct,  draining  all  of  the  rest  of  the  body.  The  area  of 
the  body  drained  by  each  system  is  represented  in  the  diagram  in 
Fig.  155. 

The  Network  of  Origin. — The  delicate  vessels  which  form  the  net- 
work of  origin  are  often  called  the  lymphatic  capillaries.  They 
resemble  the  blood  capillaries  only  in  that  their  walls  are  formed 
by  a  single  layer  of  endothelial  cells.  They  are  of  extremely  variable 
form,  depending  upon  the  character  of  the  tissue  in  which  they  are 
found.  They  form  a  very  rich  anastomosing  network  of  very  deli- 
cate vessels,  some  idea  of  the  structure  of  which  can  be  had  from 
Figs.  156  and  157.  A  few  very  delicate  vessels  collect  the  lymph 
from  this  network  and  carry  it  to  the  collecting  trunks.  The  capil- 
laries are  without  valves  but  the  collecting  vessels  are  abundantly 
supplied  with  them  (Fig.  154),  which  causes  their  characteristic 
beaded  appearance.  Stained  \vith  silver  nitrate  the  cells  are  more 
easily  outlined  than  those  of  the  blood  capillaries,  showing  cells 
30  to  40  microns  long.  Their  edges  are  wavy,  forming  lines  like  the 
sutures  of  the  skull.  Their  nuclei  are  oval  and  project  into  the 
cavity  of  the  vessel,  especially  when  they  are  not  distended.  The 
diameter  of  these  vessels  may  be  from  30  to  60  microns,  which  is 
much  greater  than  that  of  the  blood  capillaries. 

The  Collecting  Trunks. — The  walls  of  the  collecting  vessels  are 
made  up  of  three  layers:  (1)  The  endothelium.  (2)  A  layer  of 
involuntary  muscle.  (3)  An  adventitious  layer  of  white  elastic 
connective  tissue.  They  are  like  the  walls  of  the  veins,  but  more 
delicate,  less  destructible  and  more  resilient  to  pressure. 

Lymphatic  Glands  or  Lymph  Nodes. — For  the  structure  of  the 
lymph  nodes  the  student  is  referred  to  text-books  of  histology. 
They  are  by  no  means  constant  either  in  number,  size  or  position. 
In  order  to  understand  the  lymphatics  of  the  dental  region  it  is 
necessary  to  make  a  brief  statement  of  the  principal  groups  of  the 
head  and  neck  and  the  regions  which  they  drain. 

The  Lymphatics  of  the  Head  and  Neck. — The  lymphatic  glands  of 
the  head  and  neck  may  be  described  as  arranged  in  six  groups, 
forming  a  grandular  collar  or  circle  at  the  junction  of  the  head  and 


LYMPHATICS  OF  THE  HEAD  AND  NECK 


187 


neck  from  which  two  vertical  chains  extend  under  the  sterno- 
mastoid  muscle  and  along  the  large  bloodvessels  and  nerves  extend- 
ing to  where  the  neck  joins  the  thorax.  These  main  vertical  chains 
are  flanked  by  lesser  auxiliary  chains  (Fig.  158). 


.-•/.>."•••••  •^;:-'.;:':/;vi::;-'-:--' 


FIG.  156 


FIG.  157 


The  glandular  collar  is  composed  of  (1)  the  suboccipital  group; 
(2)  the  mastoid  group;  (3)  the  parotid  and  subparotid  group;  (4)  the 


188 


LYMPHATICS  OF  THE  DENTAL  REGION 


submaxillary  group;  (5)  the  submental  group;  (6)   the  retropharyn- 
geal  group. 

1.  The  suboccipital  group  usually  contains  two  glands.  They 
receive  efferents  from  the  occipital  portion  of  the  scalp.  Their 
efferents  terminate  in  the  highest  glands  of  the  substernomastoid 
group  of  the  vertical  chain. 


.. 


FIG.  158 


2.  The  Mastoid  Group. — There  are  usually  two,  one  behind  the 
other,  and  are  united  by  two  or  three  trunks.  They  lie  on  the  mas- 
toid  insertion  of  the  mastoid  muscle.  They  receive  afferents  from 
the  temporary  portion  of  the  scalp,  from  the  external  surface  of  the 


LYMPHATICS  OF  THE  HEAD  AND  NECK  189 

auricle,  except  the  lobule,  and  the  posterior  surface  of  the  external 
auditory  meatus.  Their  efferents  empty  into  the  superior  glands  of 
the  submastoid  group  after  traversing  the  superior  insertion  of  that 
muscle. 

3.  The  Parotid  Group. — This  group  is  made  up  of  (1)  the  sub- 
cutaneous glands,  which  are  often  absent;  (2)  the  glands  contained 
in  the  parotid  space;  (3)  the  subparotid  glands. 

The  glands  of  the  parotid  space  are  situated  on  the  external 
surface  of  the  gland  or  in  its  external  substance.  The  superficial 
ones  are  usually  two  or  three  in  number.  The  deep  ones  are  scattered 
through  the  entire  substance  of  the  gland  and  are  usually  grouped 
along  the  external  jugular  vein  and  the  external  carotid  artery. 
One  constantly  occupies  the  lower  part  of  the  space  and  is  attached 
close  to  the  angle  of  the  jaw  in  contact  with  the  cervical  fascia. 
They  receive  afferents  from  the  external  surface  of  the  auricle  and 
external  auditory  meatus,  from  the  tympanum,  from  the  skin  of 
the  templar  and  frontal  region,  the  eyelid  and  root  of  the  nose. 
They  perhaps  also  receive  vessels  from  the  nasal  fossa  and  the  pos- 
terior part  of  the  alveolar  border  of  the  superior  maxilla.  Their 
efferents  empty  into  the  substernomastoid  group. 

The  subparotid  glands  are  placed  between  the  parotid  and  the 
pharynx  in  the  lateropharyngeal  and  posterior  subglandular  space. 
They  are  in  contact  with  the  internal  carotid  and  the  internal 
jugular.  They  are  the  starting-point  of  the  lateropharyngeal 
abscess  (Qtiaine).  They  receive  afferents  from  the  nasal  fossa, 
nasal  pharynx  and  Eustachian  tube.  Their  efferents  pass  to  the 
glands  of  the  deep  cervical  chain. 

4.  Submaxillary  Glands. — These  glands,  three  to  six  in  number, 
are  the  most  important  from  the  dental  standpoint.    They  form  a 
chain  stretching  along  the  inferior  border  of  the  mandible  from  the 
insertion  of  the  anterior  belly  of  the  digastric  to  the  angle  of  the 
jaw.    They  are  found  in  the  junction  of  the  cutaneous  and  bony 
surface  of  the  submaxillary  gland  on  which  they  rest.    The  largest 
and  most  constant  of  the  chain  is  found  at  the  point  where  the 
facial  artery  crosses  the  border  of  the  mandible.     They  receive 
afferents  from  the  nose,  the  cheek,  the  upper  lip  and  external  part 
of  the  lower  lip,  the  anterior  third  of  the  lateral  border  of  the  tongue 
and  almost  the  whole  of  the  gums,  alveolar  process  and  teeth  of 
both  upper  and  lower  arch.    Their  efferents  descend  on  the  cutaneous 
surface  of  the  submaxillary  gland,  across  the  hyoid  bone  and  ter- 
minate in  the  glands  of  the  deep  cervical  chain,  over  the  bifurcation 


190  LYMPHATICS  OF  THE  DENTAL  REGION 

'of  the  carotid  artery  or  much  deeper,  where  the  omohyoid  crosses 
the  internal  jugular  vein. 

5.  The  Submental  Glands. — These  glands  are  extremely  variable 
in  number  and  position.    Usually  one  to  four  in  number  they  are 
found  in  the  triangle  between  the  anterior  bellies  of  the  digastric 
muscle  and  the  hyoid  bone.    They  receive  afferents  from  the  chin, 
the  central  portion  of  the  lower  lip,  the  tip  of  the  tongue  and  the 
anterior  portion  of  the  alveolar  process  and  the  lower  incisor  teeth. 
The  latter  is  probably  not  constant. 

6.  The  Retropharyngeal  Group. — These  glands  are  placed  behind 
the  pharynx  at  the  junction  of  the  posterior  and  lateral  surfaces,  at 
the  apex  of  the  lateral  masses  of  the  atlas.    Usually  two  in  number 
they  are  in  relation  with  the  posterior  wall  of  the  pharynx  and  the 
anterior  surface  of  the  rectus — capitis  anticus  major  and  externally 
with  the  constrictors  of  the  pharynx.    They  are  about  two  centi- 
meters from  the  median  line.    They  receive  afferent  vessels  from 
the  mucous  membrane  of  the  nasal  fossae  and  the  cavities  connected 
with  it,  the  nasal  pharynx,  Eustachian  tube  and  perhaps  the  tym- 
panum.   Their  efferent  vessels  empty  into  the  superior  glands  of 
the  internal  jugular  chain. 

Descending  Cervical  Chains. — These  extend  from  the  glandular 
collar  through  the  neck  to  the  thorax.  The  most  important  chain 
is  the  deep  cervical  chain,  one  on  each  side,  under  the  sternomastoid 
muscle  and  in  the  subclavian  triangle.  The  smaller  are  the  external 
jugular  chain,  the  two  anterior  cervical  chains,  superficial  and  deep, 
and  the  recurrent  chain. 

The  deep  cervical  chain  (Fig.  166)  is  one  of  the  largest  and  most 
important  relays  in  the  body.  It  contains  fifteen  to  thirty  glands. 
It  is  made  up  of  two  groups:  (1)  the  upper  or  sub  sternomastoid 
group,  and  (2)  the  lower  or  subclavian  triangular  group.  Only  the 
first  group  will  be  considered. 

Substernomastoid  Glands.  —  1.  External  Glands:  Behind  and 
external  to  the  internal  jugular  vein.  Afferent  vessels  are  received 
from  the  occipital  and  mastoid  glands  and  from  cutaneous  lym- 
phatics from  the  posterior  part  of  the  head  and  neck. 

2.  Internal  Glands:  Rest  on  the  internal  jugular  or  along  its 
external  border.  At  different  points  in  the  chain,  glands  of  special 
importance  are  found;  for  instance:  (a)  Beneath  the  posterior  belly 
of  the  digastric,  the  principal  terminus  for  lymphatics  from  the 
tongue  and  gum  about  the  lower  teeth  on  the  lingual.  (&)  "Where 
the  omohyoid  crosses  the  internal  jugular.  Afferent  vessels :  These 


LYMPHATICS  OF  THE  HEAD  AND  NECK 


191 


glands  form  the  second  relay  for  lymphs  from  the  (a)  retropharyn- 
geal  and  (&)  parotid  and  subparotid. 

3.  Submaxillary. 

4.  Submental  glands. 

5.  The  superficial  and  deep  anterior  cervical  chain  and  the  recur- 
rent chain.     They  receive  direct  afferents  from:    (a)  the  majority 
of  the  vessels  from  the  tongue;  (6)  part  of  the  nasal  pharynx  and 
larynx;  (c)  the  vault  of  the  palate  and  soft  palate;  (d)  the  cervical 
portion  of  the  esophagus;  (e)  the  nasal  fossae;  (/)  the  larynx  and 
trachea;  (g)  the  thyroid  body. 


FIG.  159 

The  Network  of  Origin  in  the  Dental  Region. — The  lymphatic 
network  of  origin  is  absolutely  continuous  over  the  whole  of  the 
face,  eyelids,  conjunctiva,  lips  and  the  mucous  membrane  of  the 
lips,  cheeks,  gums  and  gingiva.  Every  papilla  of  the  connective 
tissue  under  the  epithelium  contains  such  networks  of  vessels  as 
are  shown  in  Fig.  159  from  papilla1  of  the  hand.  Exactly  such 
structures  can  be  shown  from  the  mucous  membrane  of  the  gum 
and  gingiva1.  These  capillaries  empty  into  an  exceedingly  rich  net- 
work of  very  delicate  vessels  in  the  subcutaneous  and  submucous 
layer,  which  is  illustrated  in  Eig.  100.  It  is  difficult  for  the  element- 


192 


LYMPHATICS  OF  THE  DENTAL  REGION 


ary  student  to  get  any  conception  of  the  fineness,  delicacy  and  inter- 
communicating anastomosis  of  this  network.  From  this  network 
a  few  collecting  vessels  lead  to  the  afferent  trunks  going  to  the  first 
glands.  There  is  therefore  a  more  or  less  definite  drainage  for  a 
given  area,  though  the  network  of  origin  is  continuous. 

Lymphatics  of  the  Lips. — In  the  lips  there  are  two  networks:  one 
in  the  subcutaneous  layer  of  the  outer  surface  and  one  in  the  sub- 
mucous  layer  of  the  internal  surface.  These  communicate  freely 
at  the  border  of  the  lips.  Each  network  is  drained  by  a  few  collect- 
ing trunks,  which  receive  lymphatic  vessels  from  the  muscular 


FIG.  160. — Lymphatic  vessels  of  the  collecting  network.     (Sappey.) 


layers  that  are  less  developed.  The  subcutaneous  collecting  vessels 
of  the  lower  lip,  two  to  four  in  number  on  each  side,  frequently 
cross  and  anastomose  at  the  median  line.  Those  from  the  middle 
portion  pass  to  the  submental  glands.  Those  from  the  region  of  the 
commissure  reach  the  most  anterior  of  the  submaxillary  glands 
(Fig.  161).  The  submucous  collecting  vessels,  two  or  three  on  each 
side,  pass  obliquely  downward  and  outward  to  the  region  of  the 
facial  artery  and  end  in  the  submaxillary  glands.  They  do  not  cross 
or  anastomose  at  the  median  line.  There  are  two  submucous  and 
two  or  three  subcutaneous  collecting  vessels  in  the  upper  lip.  They 
all  pass  obliquely  downward  and  outward,  usually  to  the  middle 


LYMPHATICS  OF  THE  MOUTH  AND  GUMS 


193 


gland  of  the  submaxillary  chain.  One  of  these  may  enter  the  most 
external  of  the  collecting  trunk  from  the  lower  lip. 

Lymphatics  of  the  Mucous  Membrane  of  the  Mouth  and  Gums.- — In 
the  mucous  membrane  of  the  mouth  and  gums  the  network  of  origin 
forms  an  exceedingly  close  network. 

From  the  outer  surface  of  the  mandible  the  collecting  vessels  form  a 
wreath  of  interlacing  vessels  at  the  reflection  of  the  mucous  mem- 
brane from  the  bone  to  the  cheek.  The  vessels  increase  in  size  as 
they  pass  distally  and  finally  penetrate  the  cheek  and  end  in  the 
submaxillary  glands,  especially  the  last  one. 


FIG.  161 


From  the  inner  surface  of  the  mandible  a  similar  wreath  of  collecting 
vessels  is  formed  at  the  reflection  of  the  tissue  from  the  bone  to  the 
floor  of  the  mouth  and  tongue.  From  the  anterior  part,  lingual  to 
the  incisors,  the  vessels  pass,  with  those  from  the  tip  of  the  tongue 
to  the  submental  glands.  From  the  lateral  portion  they  unite  with 
lymphatics  from  the  anterior  part  of  the  lateral  surface  of  the  tongue 
and  pass  to  the  glands  of  the  submaxillary  chain.  From  the  region 
of  the  second  and  third  molars  they  probably  join  the  lymphatics 
from  lateral  portions  of  the  base  of  the  tongue  in  the  region  of  the 
tonsil  and  pass  to  the  large  gland  of  the  deep  cervical  chain,  placed 
under  the  posterior  belly  of  the  digastric. 

Outer  Surface  of  the  Maxilla. — From  the  outer  surface  of  the 
upper  arch  the  collecting  vessels  pass  to  a  wreath  of  large  vessels 
at  the  reflection  from  the  bone  to  the  cheek.  These  increase  in  size 
13 


194 


LYMPHATICS  OF  THE  DENTAL  REGION 


as  they  extend  distally.  At  the  level  of  the  molars  they  pierce  the 
cheek,  join  the  facial  artery  and  terminate  in  the  posterior  glands 
of  the  submaxillary  chain  (Fig.  162).  On  the  lingual  the  collecting 
vessels  first  pass  obliquely  backward  and  toward  the  median  line  of 


FIG.  162. — Lymphatic  vessels  of  the  palate.    (After  Sappey.) 

the  palate,  then  backward  and  upward  at  the  junction  of  the  hard 
and  soft  palates.  They  pass  in  front  of  the  anterior  pillar  of  the 
fauces,  pierce  the  superior  constrictor  of  the  pharynx  and  end  in  the 
large  gland  of  the  deep  cervical  chain  under  the  posterior  belly  of  the 
digastric. 


LYMPHATICS  OF  THE  PERIDENTAL  MEMBRANE       195 


Lymphatics  of  the  Peridental  Membrane. — The  lymphatic  capillaries 
in  the  papillae  under  the  epithelium  on  the  labial  or  buccal  and 
lingual  surfaces  of  the  gingivee  pass  to  the  collecting  network  in  the 
submucous  connective  tissue  outside  the  periosteum  on  the  surface 
of  the  alveolar  process  (Fig.  162).  The  lymphatic  capillaries  from 
the  papillae  under  the  epithelium  lining  the  gingival  space  are  col- 


FIG.  163. — Unstained  section,  showing  lymph  capillaries  of  the  tooth  side  of  the 
gingiviB  and  their  drainage  through  the  ligamentum  circulare  to  the  peridental 
membrane. 

lected  in  very  fine  vessels  which  pierce  the  ligamentum  circulars 
very  close  to  the  surface  of  the  cementum  and  extend  in  the  inter- 
fibrous  tissue  of  the  peridental  membrane  accompanying  the  blood- 
vessels (Fig.  163).  At  the  level  of  the  apex  of  the  root  they  receive 
lymphatics  coming  from  the  dental  pulp  (Fig.  164)  and  pass  through 
the  cancellous  spaces  of  the  bone  to  the  inferior  dental  canal  in  the 


196 


LYMPHATICS  OF  THE  DENTAL  REGION 


• 


s 


FIG.  164. — Transverse  section  just  at  the  apex  of  the  root,  showing  injected 
lymphatic  vessels  in  the  peridental  membrane  and  in  the  canals  passing  to  the 
pulp  (oc.,  2;  obj.,  16  mm.;  reduced  about  one-third). 


FIG.   10."), — DDK'S  head,  showing  lymphatic  glands  injected  from  dental  pulp. 


LYMPHATICS  OF  THE  TONGUE  197 

lower  and  the  infraorbital  canal  in  the  upper.  They  emerge  on 
the  surface  of  the  bone  at  the  mental  foramin,  or  the  infraorbital 
foramen  and  end  in  the  posterior  or  middle  glands  of  the  submaxil- 
lary  chain,  following  the  course  of  the  facial  artery  (Fig.  165).  A 
great  amount  of  work  remains  to  be  done  on  the  drainage  of  the 
teeth  in  different  regions.  Little  or  nothing  is  known  of  the  course 
of  the  vessels  from  the  upper  incisors,  lower  incisors  and  second  and 
third  molars.  Lymphatics  from  the  lower  incisors  may  pass  to  the 
submental  glands.  Those  from  the  upper  incisors  probably  reach 
the  surface  of  the  bone  below  the  level  of  the  floor  of  the  nose  and 
join  the  vessel  coming  from  the  infraorbital  canal,  though  it  is 
possible  that  some  of  them  join  vessels  in  the  floor  of  the  nose.  It 
is  quite  probable  that  lymphatics  from  the  second  and  third  molars 
pass  to  the  glands  of  the  parotid  group. 

Lymphatics  of  the  Dental  Pulp. — For  many  years  the  dental  pulp 
was  said  to  be  devoid  of  lymphatics  and  all  attempts  to  inject  vessels 
in  the  dental  pulp  failed.  In  1909  Schweitzer  reported  successful 
injections  of  the  dental  pulp,  and  in  1914  Dr.  Kaethe  Dewey  and 
the  author  repeated  Schweitzer's  results  and  succeeded  in  injecting 
lymph  capillaries  of  the  submaxillary  lymph  glands  in  the  dog  by 
injections  into  the  dental  pulp  and  followed  the  course  of  the  vessels 
continuously  from  the  pulp  to  the  glands  (Fig.  165).  There  is  much 
work  to  be  done  in  this  field  before  our  knowledge  will  be  at  all 
complete  regarding  both  the  perivascular  lymph  sheath  and  the 
independent  lymph  vessels.  The  vessels  begin  at  the  surface  of  the 
pulp  and  follow  the  course  of  the  bloodvessels  to  the  apical  foramina, 
where  they  join  the  lymphatics  of  the  peridental  membrane.  Their 
course  from  this  point  has  already  been  followed. 

Lymphatics  of  the  Tongue.1 — The  lymphatics  of  the  tongue  are 
very  highly  developed  and  have  been  thoroughly  studied.  There 
are  two  networks  of  origin:  one  superficial  in  the  mucous  membrane 
and  one  deep  in  the  muscular  body  of  the  tongue.  Their  efferent 
vessels  unite  in  the  submucosa. 

The  collecting  trunks  are  divided  into  four  groups:  (1)  Anterior 
apical.  (2)  Lateral  marginal.  (3)  Posterior  or  basal.  (4)  Median 
or  central. 

1.  Anterior  Apical  Trunks. — These  vessels,  two  on  each  side,  run 
along  the  frenum  to  the  posterior  surface  of  the  mandible.  Here 
they  separate  (Fig.  166) :  (1)  One  runs  downward  and  backward 

1  See  page  270,  The  Lymphatics  by  G.  Delarnere,  P.  Poirer  and  B,  Cuneo. 
Edited  by  Cecil  H.  Leaf. 


198 


LYMPHATICS  OF  THE  DENTAL  REGION 


between  the  geniohyoglossus  and  the  mylohyoid  crosses  the  great 
cornu  of  the  hyoid  bone  behind  the  anterior  belly  of  the  digastric 
and  along  the  external  border  of  the  omohyoid  to  the  gland  of  the 
deep  cervical  chain  where  this  muscle  crosses  the  internal  jugular 
vein.  (The  general  statement  is  that  the  more  anterior  the  origin 
in  the  tongue  the  lower  the  gland  in  the  deep  cervical  chain  to  which 
it  goes.)  (2)  The  second  trunk  passes  to  the  submental  gland. 


FIG.  166 

2.  The  Marginal    Trunks. — These  vessels   collect   from  all  the 
mucous  membrane  from  the  tip   of  the  tongue  to  the  V-shaped 
groove  on  the  dorsal  surface.    They  are  eight  to  twelve  in  number: 
(1)  One  group,  the  external  (three  or  four),  pierce  the  mylohyoid 
and  pass  around  the  inferior  border  of  the  mandible  to  the  glands 
of  the  submaxillary  chain.     (2)  The  internal  (five  or  six).    These 
vessels  run  downward  and  backward  on  the  muscles  of  the  tongue 
and  all  end  in  glands  of  the  deep  cervical  chain. 

3.  Basal    Trunks. — These  vessels    (seven  or   eight)    arise  from 
the  region  of  the  circurnvallate  papillae  and  are  the  largest  and 


LYMPHATICS  OF  THE  TONGUE  199 

most  important  vessels  of  the  tongue.  They  form  a  medial  and 
lateral  group  and  all  terminate  in  the  large  gland  of  the  deep  cervical 
under  the  posterior  belly  of  the  digastric. 

4.  The  Central  Trunks. — These  vessels  arise  from  the  middle  part 
of  the  dorsal  network  of  the  body  of  the  tongue.  Instead  of  running 
outward  they  descend  in  the  middle  line  between  the  two  genio- 
hyoglossi  and  end  in  the  glands  of  the  deep  cervical  chain. 


CHAPTER  XV. 
INTERCELLULAR  SUBSTANCES. 

DURING  the  last  hundred  years,  knowledge  of  living  things  and 
all  thought  of  their  structure  and  function  has  entirely  changed. 
The  cell  theory  has  abundantly  established  that  the  cell  is  the 
structural  and  functional  unit  of  all  living  objects,  both  plant  and 
animal,  and  that  all  manifestations  of  life  are  accomplished  by  the 
chemical  activity  of  the  substance  of  the  cell,  which  Huxley  long 
ago  designated  as  "the  physical  basis  of  life."  From  a  considera- 
tion of  the  physical  properties  of  cytoplasm,  nothing  is  more  appar- 
ent than  that  the  production  of  a  highly  organized  body  out  of 
it  alone  would  be  impossible.  If  the  human  body  were  composed 
entirely  of  cytoplasm  it  would  be  a  shapeless  lump  of  jelly.  It 
is  only  by  the  production  of  material  which  has  physical  properties 
of  strength  and  rigidity  through  the  activity  of  the  cytoplasm  that 
the  shape  and  function  of  a  highly  organized  creature  is  possible. 
This  is  accomplished  through  the  metabolism  of  the  cytoplasm 
more  or  less  analogous  to  the  building  up  of  a  secretion  by  the  cells 
of  a  gland,  though  there  is  no  intention  to  suggest  any  direct  com- 
parison between  the  two.  In  other  words,  all  tissues  are  made  up 
of  cells  and  intercellular  substance,  and  the  vital  characteristics 
are  given  to  the  tissue  by  the  cells,  the  physical  characteristics  by 
the  intercellular  substance.  These  intercellular  or  extracellular 
materials  possess  none  of  the  vital  manifestations,  and  are  entirely 
dependent  upon  the  cells  for  their  formation  and  maintenance. 
There  is  apparently  a  constant  reaction  between  the  cell  and  the 
formed  material  which  constitutes  the  intercellular  substance,  for 
even  the  most  highly  specialized  of  intercellular  substances  repre- 
sented by  the  dentin  matrix  changes  in  its  properties  if  the  cells 
are  removed.  If  the  cells  in  the  bone  matrix  are  killed,  that  portion 
of  the  tissue  becomes  necrosed  bone  and  is  as  much  a  piece  of 
foreign  matter  as  if  a  piece  of  bone  toothbrush  handle  had  been 
shot  into  the  body.  The  fibers  of  fibrous  tissue  have  no  ability  to 
grow,  to  attach  themselves  to  any  surface,  or  even  to  maintain 
their  present  form  without  the  presence  of  living  cells  or  fibroblasts. 
( 200 ) 


INTERCELLULAR  SUBSTANCES  201 

There  has  been  a  great  deal  of  discussion  as  to  the  method  of  forma- 
tion of  intercellular  substances  by  the  cells,  and  the  nature  of  the 
reaction  occurring  between  the  cell  and  the  formed  material  after 
it  has  been  produced.  In  several  intercellular  substances  the 
material  passes  through  changes  both  of  physical  and  of  chemical 
character,  but  these  are  carried  out  by  reaction  with  materials 
formed  by  the  metabolism  of  the  cell,  for  if  the  cells  are  removed 
the  formed  material  will  not  go  through  any  such  changes.  The 
intercellular  substances,  therefore,  while  they  are  chemically 
extremely  complex,  belong  to  the  simplest  classes  of  protein  mole- 
cules, and  have  no  such  complexity  of  atomic  movement,  producing 
conditions  of  recurrent  unsatisfied  affinity,  without  which  no  idea 
of  the  metabolism  of  living  cytoplasm  can  be  obtained.  Chemically, 
living  cytoplasm  may  be  roughly  viewed  as  constantly  undergoing 
chemical  changes  which  are  almost  infinitely  complex,  and  by  means 
of  which  simpler  substances  are  acted  upon  and  built  into  its  own 
molecule.  Complex  combinations  are  thrown  off  as  products  of 
its  metabolism,  and  simpler  substances  are  formed  as  decomposi- 
tion products,  or  waste  materials.  Dr.  Brooks  often  used  to  say 
in  his  lectures  that  the  most  striking  characteristic  of  living  things 
was  their  ability  to  react  upon  their  environment  in  such  a  way 
as  to  become  better  and  better  suited  to  it.  When  living  cytoplasm, 
which  is  soft  and  without  the  physical  properties  of  strength  and 
rigidity,  requires  protection  from  physical  influences,  substances 
possessing  these  qualities  are  produced  by  it.  Intercellular  sub- 
stances therefore  were  apparently  formed  by  the  cytoplasm  in 
response  to  physical  conditions  of  its  environment,  and  are  one  of 
the  phases  of  adaptation. 

In  the  higher  forms  of  animal  life  the  class  of  tissues  which 
have  produced  these  formed  materials,  for  the  purpose  of  support, 
rigidity,  and  connection,  are  called  the  connective  or  supporting 
tissue.  The  formed  materials  are  of  two  classes — those  which  are 
to  connect  associated  and  dependent  parts,  and  those  which  give 
rigidity  and  protection.  The  fibrous  tissues  are  of  the  first  class, 
and  are  made  up  of  materials  possessing  strength  and  elasticity. 
The  bone  and  cartilage  belong  to  the  second  class,  and  give  strength 
and  rigidity.  The  first  sustain  pulling  stress,  the  latter  shearing 
or  bending  stress,  though  both  possess  a  certain  amount  of  each. 

Adaptability  and  the  greatest  range  of  variation  are  most  strik- 
ing characteristics  of  connective  tissue  which  develop  and  change 
to  meet  all  kinds  of  requirements  of  both  mechanical  and  physical 


202 


INTERCELLULAR  SUBSTANCES 


environment  to  which  they  are  subjected.  These  variations  are 
produced  by  the  production  of  increased  amount  of  the  intercellular 
material,  its  destruction,  or  the  change  of  its  character,  under  the 
influence  of  the  cells  of  the  tissue.  No  tissue  responds  more  quickly 
to  the  demands  made  upon  it  by  development  or  environment. 
When  the  muscles  grow  larger  and  stronger  by  development,  the 
tendons  and  the  bones  to  which  they  are  attached  change  as  quickly 
and  in  proportion.  From  the  appearance  of  the  skeleton  the 
experienced  anatomist  can  picture  very  accurately  the  muscular 
development  of  the  individual  to  whom  it  belonged. 

The  cell  wall  of  plants  may  be  used  as  one  of  the  simplest  examples 
of  supporting  tissue.    In  this  case  each  cell,  in  addition  to  its  other 


FIG.  167. — Cells  from  the  growing  tip  of  a  chestnut  seedling.     (Dahlgren  and 

Hepner.) 

functions,  produces  its  own  supporting  substance.  These  may  be 
observed  in  the  cells  of  a  growing  root  tip.  Plant  an  onion,  by 
selecting  one  larger  than  a  small  glass,  fill  the  glass  with  water, 
and  place  the  bulb  on  it.  If  this  is  placed  in  a  sunny  window,  in 
a  few  hours  little  rootlets  will  be  seen  stretching  down  into  the 
water.  The  rootlets  of  a  sprouting  chestnut  also  make  very  good 
material  (Fig.  167).  If  these  are  embedded  in  paraffin,  the  develop- 
ment of  the  cells  and  the  formation  of  their  supporting  walls  can  be 
observed.  The  young  cells  near  the  tip  will  be  found  to  be  a  mass 
of  granular  cytoplasm,  with  a  large  nucleus  in  the  center,  and  a 
thin  wall  of  cellulose  which  is  the  cell  organ  of  support.  As  the 
cell  increases  in  size,  vacuoles  appear  in  the  cytoplasm  which  become 


INTERCELLULAR  SUBSTANCES  203 

larger  and  larger.  These  vacuoles  are  filled  with  watery  fluid  which 
is  not  a  part  of  the  cytoplasm.  If  the  cell  remained  a  solid  mass 
of  cytoplasm,  an  enormous  amount  of  food  material  would  be 
required,  which  would  be  out  of  all  proportion  to  the  work  which 
the  cell  is  to  perform.  The  vacuoles  increase  in  size  with  the 
growth  of  the  cell  until  there  is  a  rim  of  cytoplasm  in  contact  with 
the  cell  wall,  and  a  central  mass  of  cytoplasm  surrounding  the 
nucleus  and  connected  with  that  at  the  periphery  by  fine  threads. 
In  still  further  growth  these  threads  are  broken,  the  nucleus  is 
pushed  to  one  side,  and  the  whole  central  portion  becomes  one 
huge  vacuole.  There  is  now  a  cell  wall,  with  a  layer  of  cytoplasm 
covering  its  inner  surface,  which  is  kept  in  reaction  with  the  nucleus 
by  streaming  around  and  around.  This  flowing  of  the  cytoplasm  in 
plant  cells  may  be  easily  observed  in  the  delicate  stamen  hairs  of 
the  ordinary  Spiderwort,  or  in  the  cells  of  the  water  plants  Chara 
or  Nitella,  which  are  easily  found  in  most  ponds.  In  this  example 
it  is  seen  that  the  cytoplasm  remains  in  contact  with  the  formed 
material  which  it  produces  for  support,  and  that  it  is  only  sufficient 
in  amount  to  form  and  maintain  this  material. 

In  general  histology  it  has  already  been  noted  that  the  cells  of 
connective  tissue  are  very  similar,  and  that  the  tissues  differ  chiefly 
in  the  character  and  arrangement  of  the  intercellular  substances. 
It  has  also  been  emphasized  that  the  connective  tissues  all  originate 
from  a  common  form  of  embryonal  connective  tissue,  or  mesen- 
chyme,  and  change  from  one  form  to  another  in  development. 
These  mutations  of  the  connective  tissues  are  their  most  striking 
characteristic,  and  must  be  clearly  grasped  if  the  bone,  as  an  organ 
of  support,  is  to  be  understood.  For  instance,  embryonal  connec- 
tive tissue  is  transformed  into  fibrous  tissue;  fibrous  tissue  becomes 
arranged  in  a  definite  membrane,  and  is  transformed  into  cartilage, 
which  is  again  removed  and  transformed  into  bone.  All  these 
changes  take  place  to  meet  the  requirement  of  mechanical  conditions 
and  environment. 

If  the  subcutaneous  tissue  of  an  embryo  be  examined  in  sections 
(Figs.  168  to  183)  the  cells  will  be  found  to  be  irregular  masses  of 
cytoplasm  with  a  nucleus  in  the  central  portion,  and  fine  projec- 
tions stretching  out  in  all  directions  through  an  almost  structureless 
intercellular  substance.  The  fine  projections  of  the  cytoplasm  meet 
those  of  the  adjoining  cells  and  form  a  network  holding  everything 
together.  Because  of  the  nature  of  cytoplasm,  however,  these 
possess  very  little  strength,  and  very  soon  fine  thread-like  fibers 


204 


INTERCELLULAR  SUBSTANCES 


are  found  appearing  in  the  intercellular  substance  in  contact  with 
cells.     These  unite  with  each  other,  forming  continuous  fibers, 


FIG.      168. — Embryonal      connective  FIG.    169. — The  same,   a    little  more 

tissue  in  an  early  stage  of  development,  developed,  showing  the  cellular  elements 

showing  the  cellular  elements  embedded  lengthening    in    a    common     direction, 

in  the  ground  substance.     (Black.)  (Black.) 


-^;^^;r^.^-^^^3 

\  ^-^Trr^T^^T^TTTc^T^i^- 

W"         — ^&^-:J'^-^^ 


FIG.    170. — The  cells  developed   in  spindle  forms,  fibroblasts   with    long  filaments 
extending  from  either  end.     (Black.) 


FIG.  171. — The  developed  white  fibrous  tissue.    (Black.) 


FIG.  172. — Older  white  fibrous  tissue,  in  which  the  cells  are  no  longer    seen,  and 
showing  the  wave-like  course  of  the  fibers.    (Black.) 

and  very  soon  a  strong  network  is  produced  which  is  entirely  depen- 
dent upon  the  cytoplasm  of  the  cell  which  has  formed  and  main- 


INTERCELLULAR  SUBSTANCES 


205 


tains  it.  If  this  tissue  is  now  subjected  to  pressure  and  strain, 
the  cells  become  flattened  out  and  squeezed  between  the  bundles 
of  fibers,  which  take  on  parallel  directions,  and  so  a  tendon  is 


FIG.  173. — Coarse  white  fibers,  made  up  of  bundles  of  the  fine  fibers,  and  showing  the 
mode  of  division  by  splitting  off  of  a  portion  of  the  fibers  of  the  bundle.     (Black.) 


FIG.  174. — Coarse  fiber  breaking  up  into  fine  fibers.     (Black.) 


FIG.  175. — Cross-sections  of  coarse  fibers,  showing  some  of  their  various  forms. 

(Black.) 


FIG.  177.  —  Cross-sec- 
FIG.    176. — Reticular   or    elastic    fibers,   showing    the        tions     of     the     reticular 
mode  of  division  and  the  multipolar,  or  irregular,  star        fibers,  showing    some  of 
forms  of  the  cells  at  the  divisions.     (Black.)  their  forms.      (Black,) 


206 


INTERCELLULAR  SUBSTANCES 


formed.  A  tendon  must  be  considered  as  a  highly  specialized 
form  of  connective  tissue,  arranged  to  supply  tensile  strength. 
The  degree  of  specialization  of  the  tissue  is  judged  by  the  extent  to 


FIG.  178. — Connective-tissue  cells  from  which  reticular  fibers  are  developed. 

(Black.) 


FIG.   179.— Network  of  elastic  fibers  FIG.   180. — Network  of  elastic  fibers 

from  the  point  of  reflection  of  the  mu-  teased    out    from    elastic    tendon,    and 

cous  membrane  of  the  lip  from  the  gums.  showing    the    usual    mode   of    division. 

(Black.)  (Black.) 


which  its  characteristic  features  are  developed,  either  in  quantity 
or  quality.  In  the  tendon  the  fine,  strong  fibers  have  been  gathered 
into  bundles;  a  round  nucleus  would  occupy  too  much  space. 


INTERCELLULAR  SUBSTANCES 


207 


It  has  therefore  become  elongated  and  more  or  less  rod-shaped, 
and  the  cytoplasm  has  been  squeezed  out  into  thin  leaf-like  projec- 
tions between  the  bundles.  Each  cell  is  in  contact  with  several 
fibers,  and  each  fiber  in  contact  with  the  cytoplasm  of  cells  which 
have  produced  them. 


FIG.   181. — Elastic  fibers,  showing  their  disposition  to  curl  up  when  cut  or  broken. 

(Black.) 

It  must  be  supposed  that  there  is  a  constant  reaction  between 
the  substance  of  the  formed  material  and  materials  produced  by  the 
metabolism  of  the  cytoplasm.  In  pathologic  conditions  the  metab- 
olism of  the  cytoplasm  is  disturbed,  and  there  is  a  consequent 
change  in  the  quality  of  the  fibers.  So  in  some  pathologic  condi- 
tions a  relaxation  and  loss  of  tone  is  found  in  tendons  and  ligaments. 
In  inflammations  of  the  gingivse  the  fibers  become  relaxed  and 
stretched,  so  that  the  gingivse  are  everted,  but  return  to  their 
normal  condition  when  the  pathologic  condition  has  subsided,  and 
the  cells  regain  their  normal  metabolism. 


FIG.  182. — Cross-sections  of  elastic 
fibers,  showing  their  forms  as  seen  in 
a  group  passing  between  coarse  white 
fibers.  (Black.) 


FIG.  183. — Tissue  of  the  dental  pulp,  in 
which  the  development  of  the  cells  is  not 
followed  by  any  considerable  formation  of 
fibers.  (Black.) 


To  sum  up  what  has  been  said,  it  is  apparent  that  both  phylo- 
genetically  and  ontogenetically,  intercellular  substances  have 
been  produced  and  are  maintained  by  cells  in  response  to  mechanical 
influences  and  to  meet  mechanical  conditions.  In  all  higher  animals 
certain  tissues,  the  connective  tissues,  have  been  set  apart  for  this 
purpose,  and  the  cells  have  been  specialized  to  respond  to  mechan- 


208  INTERCELLULAR  SUBSTANCES 

ical  stimuli  and  develop  an  intercellular  substance  adapted  to  the 
condition.  This  makes  the  supposition  necessary  that  an  embryonal 
connective-tissue  cell  may  develop  into  any  specialized  form  and 
that  the  kind  of  cell  into  which  it  develops  will  be  determined  by 
the  character  of  mechanical  stimuli  which  it  receives.  Just  as  the 
epithelial  cells  have  been  specialized  to  respond  to  the  environments 
of  light  stimuli,  vibration  of  the  air,  pressure,  and  chemical  action 
which  connect  the  organism  with  its  environment,  connective- 
tissue  cells  have  been  specialized  to  respond  to  mechanical  stimuli,, 
by  the  production  of  formed  materials  adapted  to  the  mechanical 
conditions.  These  conceptions  are  fundamental  to  an  understand- 
ing of  bone  structure  and  growth,  and  the  mutations  of  connective 
tissue  in  general. 

In  no  branch  of  histology  is  a  clear  conception  of  intercellular 
substances  and  the  relation  of  cells  to  them  as  important  as  in  the 
study  of  the  teeth  and  their  associated  structures.  Caries  cannot 
be  understood  unless  these  fundamental  ideas  have  been  appreciated, 
and  many  statements  in  dental  literature  would  never  have  appeared 
if  the  nature  of  intercellular  substance  and  the  relation  of  cytoplasm 
so  it  had  been  understood. 


CHAPTER  XVI. 
BONE. 

Definition. — Bone  may  be  defined  as  a  connective  tissue  whose 
intercellular  substance  is  calcified  and  arranged  in  layers  around 
nutrient  canals  or  spaces.  The  cells  are  placed  in  cavities,  lacunae, 
between  the  layers,  and  receive  their  nourishment  through  very 
minute  channels,  canaliculi,  which  radiate  from  them  and  penetrate 
the  layers. 

STRUCTURAL   ELEMENTS. 

The  structural  elements  of  bone  are: 

1.  Bone  matrix,  or  intercellular  substance,  which  is  always 
arranged  in  layers  or  lamellae. 


FIG.  184. — From  a  section  through  the  bone  of  a  roebuck.     The  lacunae  are  seen 
from  above,  and  are  filled  with  coloring  matter.     In  places  small  dots  are  visible, 
which  represent  the  cross-sections  of  bone  canaliculi.     (850  X)     (Szymonowicz.) 
14  (209) 


210 


BONE 


2.  The  bone  cells  or  bone  corpuscles  which  are  embedded  in  the 
matrix  between  its  layers. 

3.  Lacunae,  or  the  spaces  in  which  the  cells  are  found. 

4.  Canaliculi,  or  the  channels  through  the  matrix  by  which 
the  embedded  cells  receive  nourishment. 

Bone  Matrix. — The  bone  matrix  is  composed  of  a  dense  organic 
basis  of  ultimately  fibrous  character  which  yields  gelatin  upon 


FIG.  185. — From  a  section  through  the  bone  of  a  roebuck.     The  lacuna;  are  seen  from 
the  side.     (850  X)     (Szymonowicz.) 


boiling  with  water.  With  this  inorganic  salts  are  combined  in  a 
weak  chemical  union,  forming  the  hard  substance  of  bone.  By 
treatment  with  acids  the  inorganic  salts  can  be  removed,  leaving 
the  organic  basis  which  retains  the  form  of  the  tissue.  In  this 
condition  the  rigidity  of  the  bone  is  destroyed.  On  the  other  hand, 
by  calcining  at  red  heat  the  organic  basis  can  be  removed,  leaving 
the  inorganic  substances  which  retain  the  form  of  the  tissue.  In 
formation  the  organic  basis  is  apparently  formed  first,  and  then 


HAVE  RSI  AN  SYSTEM  BONE  211 

the  salts  of  lime  are  combined  with  it,  through  the  agency  of  the 
formative  cells  or  osteoblasts. . 

Bone  Corpuscles. — Bone  corpuscles  are  the  cells  lying  in  the 
lacunae.  Each  cell  contains  a  single  well-defined  nucleus,  lying  in 
the  centre  of  a  granular  cytoplasm.  The  cell  apparently  completely 
occupies  the  lacunae,  and  from  the  central  mass  fine  projections  of 
cytoplasm  extend  through  the  canaliculi,  which  bring  the  bone 
corpuscles  in  intimate  relation  with  certain  areas  of  bone  matrix. 
The  processes  of  one  cell  anastomose  with  those  of  its  neighbors 
through  the  canaliculi,  so  that  there  is  a  continuous  network  of 
living  cytoplasm  throughout  the  matrix. 

Lacunse. — The  lacunas  are  flat,  oval  spaces  about  20  microns 
long,  10  microns  wide,  and  5  or  6  microns  thick.  Their  shape, 
therefore,  in  sections  depends  upon  the  way  in  which  they  are  cut, 
as  illustrated  in  Figs.  184  and  185.  When  cut  lengthwise  they  would 
appear  as  about  20  microns  long  and  6  wide  in  profile,  or  as  about 
20  microns  long  and  10  wide  when  seen  from  above. 

Canaliculi. — These  radiate  from  the  lacunse  in  all  directions, 
opening  into  them  by  larger  channels  which  branch  and  divide, 
becoming  smaller  as  they  pass  farther  into  the  matrix.  They 
anastomose  freely  with  those  from  adjoining  lacunae. 

THE    VARIETIES    OF   BONE. 

There  are  three  varieties  of  bone  differing  in  the  arrangement 
of  these  structural  elements.  These  are  subperiosteal,  Haversian 
system,  and  cancellous  bone. 

Subperiosteal  Bone. — This  form  of  bone  must  be  regarded  as 
primarily  a  formative  arrangement  and  more  or  less  transitory, 
in  which  the  layers  are  arranged  parallel  with  the  surface,  and 
under  a  formative  membrane.  It  contains  canals  (Volkmann's 
canals)  with  bloodvessels  (Fig.  186),  connective  tissue,  etc.  These 
penetrate  the  layers  which  are  never  arranged  concentrically  around 
them.  It  is  always  thin,  that  is,  composed  of  comparatively  few 
layers,  and  when  a  considerable  thickness  is  formed  it  is  cut  out 
from  within  by  absorptions  beginning  in  the  canals,  and  bone  is 
rebuilt  with  layers  arranged  concentrically  around  the  channels 
formed.  In  this  way  subperiosteal  bone  is  converted  into  the 
second  form. 

Haversian  System  Bone. — In  this  variety  the  lamellae  are  arranged 
concentrically  around  canals  which  contain  bloodvessels,  nerves, 


212 


BONE 


and  embryonal  connective  tissue,  and  from  which  the  cells  in  the 
lacunae  are  nourished  (Fig.  187).  These  canals  are,  in  general, 
parallel  with  the  surface  or  the  long  axis  of  the  bone  and  anastomose 
with  each  other.  A  canal  with  the  layers  arranged  around  it  con- 
stitute a  Haversian  system.  Between  the  Haversian  systems  are 
remains  of  the  subperiosteal  layers  (interstitial  lamellae)  that  were 
left  by  the  absorption,  and  for  that  reason  have  been  called  fun- 
damental lamellae.  They  have  also  been  called  ground  lamellae. 
Haversian  system  bone  is  often  called  compact  bone,  and  makes  up 


FIG.    186. — Subperiosteal    bone, 
showing  Volkmann's  canals. 


FIG.  187. — Haversian  system  bone: 
Haversian  canals. 


the  greater  part  of  the  shafts  of  the  long  bone,  and  the  plates  of 
the  flat  ones.  It  is  never  allowed  to  become  greater  in  thickness 
than  is  necessary  for  strength,  and  when  sufficient  thickness  has 
been  formed,  the  deeper  part  is  cut  out  by  absorptions  in  the 
Haversian  canals,  converting  them  into  large  irregular  spaces. 
The  formation  of  a  few  layers  around  these  spaces  transforms  the 
second  type  into  the  third  or  cancellous  bone. 

Cancellous  Bone. — In  this  variety  the  lamellae  are  arranged  in 
delicate  plates  surrounding  large,  irregular  nutrient  or  marrow 
spaces.  These  are  filled  by  embryonal  connective  tissue  and  con- 


COMPACT  BONE  213 

tain  bloodvessels  and  nerves.  The  plates  of  cancellous  bone  are 
not  arranged  at  haphazard,  as  might  be  supposed  from  a  casual 
observer  of  sections,  but  are  disposed  in  definite  arrangement, 
which  is  determined  by  the  directions  of  stress  on  the  compact 
bone  which  they  support.  (See  illustrations  in  Chapter  XXVII.) 
They  are  not  permanent  and  unchanging,  but  are  continually  being 
rebuilt  in  new  directions,  in  response  to  the  mechanical  conditions 
to  which  the  bone  as  a  supporting  organ  is  subjected. 

THE  ARRANGEMENT   OF   BONE. 

Compact  Bone. — A  knowledge  of  the  structural  elements  of  bone 
can  best  be  obtained  by  the  study  of  sections  ground  from  the 
shaft  of  a  long  bone.  An  old  dry  bone  should  be  sawed  across, 
near  the  middle  of  the  shaft,  in  two  places,  so  as  to  cut  out  a  ring 
about  a  quarter  of  an  inch  thick.  Then  saw  the  ring  through  in 
two  places  with  an  arc  of  about  a  quarter  of  an  inch  on  the  outer 
surface.  From  this  twro  slices  should  be  sawed  out,  one  transverse 
to  the  long  axis  of  the  bone,  the  other  parallel  with  it.  These 
are  ground  to  not  more  than  8  or  10  microns  in  thickness  and 
mounted  in  hard  balsam.  From  a  study  of  these  two  the  arrange- 
ment of  the  lamellae,  and  the  shape  and  character  of  the  lacunse 
can  be  made  out.  Upon  the  outer  surface  of  the  transverse  section 
will  be  found  a  larger  or  a  smaller  number  of  layers  of  subperiosteal 
bone  which  encircle  the  shaft,  and  consequently  are  called  the 
circumferential  lamellae.  The  number  of  these  layers  will  depend 
upon  the  position  from  which  the  section  is  taken,  and  the  age  of 
the  bone.  If  the  bone  is  increasing  in  circumference  at  the  point 
from  which  the  section  is  cut,  there  will  be  a  considerable  number 
of  layers,  and  they  will  be  easily  seen.  If  the  bone  has  been  growing 
smaller  in  circumference  at  the  point,  there  will  be  very  little  of 
subperiosteal  bone,  and  it  will  be  comparatively  hard  to  recognize. 
The  greatest  part  of  the  section  will  be  made  up  of  Haversian  sys- 
tems, in  which  from  two  or  three  to  five  or  six  layers  are  arranged 
around  an  Haversian  canal.  The  lacunas  appear  as  irregularly  oval 
spaces  about  5  or  6  microns  across  and  15  to  20  microns  in  length. 
From  them  a  great  many  minute  canals  radiate  through  the  matrix, 
both  toward  the  Haversian  canal  and  away  from  it.  The  character 
of  these  canaliculi  can  only  be  appreciated  by  seeing  them.  They  are 
filled  in  life  by  projections  of  the  protoplasm  of  the  bone  corpuscles. 
They  are  suggestive  of  the  rootlets  of  plants  running  through  the 


214  BONE 

soil,  and  as  in  that  case  the  rootlets  are  absorbing  material  from 
the  soil  and  reacting  with  it,  in  this  case  the  protoplasmic  contents 
of  the  canaliculi  are  reacting  with  the  matrix,  maintaining  its  quality. 
The  portion  of  matrix  through  which  the  canaliculi  from  one  lacunae 
extend  belongs  to  the  bone  corpuscles  which  occupies  the  lacunae, 
as  will  be  seen  later.  These  cells  have  been  enclosed  in  the  matrix 
which  they  have  formed.  Between  the  Haversian  systems  will 
be  found  a  few  layers  of  interstitial  or  fundamental  lamellae.  They 
are  the  remains  of  layers  which  were  formed  under  the  periosteum 
and  were  not  entirely  destroyed  when  it  was  replaced  by  Haversian 
systems  (Plate  XIII).  The  amount  of  interstitial  lamellae  varies 
greatly  in  different  specimens,  as  will  be  seen  by  comparing  figures. 

The  Haversian  canals  anastomose  with  each  other;  this  will  be 
seen  in  many  specimens.  Many  Haversian  systems  will  be  found 
imperfect  in  form,  as,  for  instance,  those  shown  in  Plate  XIII.  This 
means  that  after  these  systems  were  completed,  absorptions  occurred 
in  a  neighboring  canal  which  attacked  the  layers  of  the  system,  and 
later  a  new  system  was  formed  in  this  space  by  the  deposit  of 
concentric  lamellae.  While  bone  is  thought  of  as  a  hard  and  fixed 
tissue,  it  is  continually  being  built  and  rebuilt  in  this  way.  It  is 
only  by  the  understanding  of  these  possibilities  that  we  get  the 
ideas  that  bone,  while  hard  and  rigid,  is  a  plastic  tissue  and  is  con- 
tinually being  moulded  by  mechanical  conditions  to  which  it  is 
subjected. 

It  will  be  seen  also  that  the  arrangement  of  the  lamellae  becomes 
a  record  of  the  changes  that  have  occurred  in  the  formation  of  the 
tissue.  The  inner  boundary  of  the  section  next  to  the  marrow 
cavity  will  show  a  few  layers  parallel  with  the  surface.  These  are 
known  as  the  inner  circumferential  lamellae.  It  is  a  mistake,  how- 
ever, to  think  of  them  as  surrounding  the  marrow  cavity  in  the 
same  sense  as  the  outer  circumferential  lamellae  surround  the  bone. 
If  the  section  has  been  cut  at  a  little  distance  from  the  center  of 
the  shaft,  it  will  have  been  noted  that  the  marrow  cavity  is  pene- 
trated by  very  delicate  spicules,  and  that  in  fact  the  marrow  cavity 
is  produced  by  the  spaces  of  cancellous  bone,  becoming  larger  and 
larger  until  they  become  one  continuous  space.  The  inner  circum- 
ferential lamellae  are  therefore  the  layers  which  have  been  formed 
around  an  enlarged  nutrient  or  marrow  space. 

Cancellous  Bone. — The  cancellous  bone  can  best  be  studied  in 
decalcified  sections.  A  field  from  the  central  portion  of  a  flat 
bone  will  show  its  typical  arrangement.  It  is  made  up  of  delicate 


PLATE  XIII 


V1'    -^+''"L  ~    -  -    *-  *  '    *•      •  •<    -v^ 
"  "  -".    _      *•  ^   -  - 


*V\*  ,V -:: :  3*  :&  ^^^  '  -  f-  ^\^v^  •     -  v 

Kv^^?^^^  :;x^- 


sXd 


^     k     '  '  i     *  "  *"?••-      '      '   i  '     '     *   t              ^F   '  *       ' 

'                l/X'.t:--  U  -^         '     ,    '»     ®      ^       '' -    *    ^       ^ 

i  j  '      '  ^^  »t  * '  ;r  ^     x  5.     /  M    ,»   v .  *  ••  ^  •**'  ^          »  4* 

^  .  "  ^'T',!  "*,'  >'  ""    ".   :^ ..  '•*      'v  y  "       ^''-!^'','-^";:i-'-^ 


,x  '  /  / 

'•"  *  J£* 

-^% 


'f.7 
'  /.  I 


From  a  Ground  Cross-section  of  the  Diaphysis  of  the 
Human  Metatarsus.     (Szymonowicz.) 

<7,  outer  ground  lamellae;  h.  inner  groi\ncl  lamellae;  c,  Haversian  lamellae; 
<l.  interstitial  lamellae.  All  canals  and  bone  cavities  are  filled  with  coloring 
matter  and  appear  black.  (9O  X-) 


CANCELLOUS  BONE  215 

flattened  spicules  surrounding  larger  or  smaller  irregular  spaces 
which  connect  with  each  other  very  freely.  Each  spicule  is  com- 
posed of  a  few  lamellae  which  are  arranged  around  the  space.  The 
structure  of  the  spicules  often  becomes  complicated  by  absorp- 
tions and  rebuildings  which  have  occurred  to  change  their  direc- 
tion. The  tissue  which  fills  the  spaces  is  a  delicate,  embryonal 
connective  tissue  in  which  osteoblasts  and  osteoclasts  appear  in 
response  to  mechanical  conditions.  It  is  richly  supplied  with 
bloodvessels,  nerves,  and  lymphatics.  The  lacunas  and  canaliculi 
are  in  no  respect  different  from  those  of  the  Haversian  system  and 
subperiosteal  bone. 


CHAPTER  XVII. 
BONE  FORMATION  AND  GROWTH. 

BONE  is  one  of  the  latest  tissues  to  be  formed,  and  is  always 
developed  from  an  antecedent  connective  tissue  of  less  specialized 
character.  According  to  the  character  of  the  antecedent  tissue 
bone  formation  is  of  two  varieties — the  formation  from  cartilage, 
or  endochondral  bone  formation,  and  that  from  fibrous  connective 
tissue,  without  the  intervention  of  cartilage,  or  endomembranous 
bone  formation. 

Endochondral  Bone  Formation. — All  of  the  bones  of  the  endo- 
skeleton  are  preformed  in  cartilage.  The  transformation  of  car- 
tilage into  bone  is  rather  a  substitution  than  a  transformation, 
for  the  original  tissue  is  destroyed  in  the  process,  and  a  new  and 
more  highly  specialized  one  substituted  for  it. 

Before  ossification  begins  the  cartilage  has  taken  on  the  general 
form  of  the  bone  and  is  covered  by  a  definite  perichondrium.  Ossi- 
fication begins  at  separate  centers  and  progresses  through  the  carti- 
lage, but  the  separate  centers  do  not  unite  until  the  bone  is  about 
fully  formed.  In  the  long  bone  there  are  usually  three  centers — 
one  near  the  center  of  the  shaft,  forming  the  hypophysis,  and  one 
near  either  end,  forming  the  epiphyses.  These  remain  separated 
by  a  layer  of  cartilage  until  the  length  of  the  bone  has  been  fully 
formed. 

The  first  indication  of  the  transformation  of  cartilage  into  bone 
is  an  increase  in  the  size  of  the  lacunae  and  in  the  amount  of  cartilage 
matrix,  which  also  shows  changes  in  character,  having  lime  salts 
deposited  in  it.  The  cartilage  cells  enlarge  and  show  signs  of 
degeneration,  the  lacunae  become  arranged  in  row7s,  and  as  they 
increase  in  size,  more  in  the  direction  parallel  writh  the  axis  of  the 
cartilage,  the  amount  of  matrix  separating  them  is  reduced.  By 
this  time  the  perichondrium,  on  the  surface  of  the  cartilage  opposite 
to  the  center,  has  developed  osteoblasts  which  begin  the  formation 
of  subperiosteal  lamellae  upon  the  surface  of  the  cartilage,  and  the 
perichondrium  is  transformed  into  periosteum.  Opposite  the 
center  osteoclasts  appear,  cutting  into  the  cartilage,  followed  by 
(216) 


ENDOCHONDRAL  BONE  FORMATION 


217 


buds  of  embryonal  tissue.  The  osteoclasts  dissolve  away  the 
remains  of  the  cartilage  matrix,  opening  up  the  spaces  between  the 
lacunae  and  converting  the  rows  of  lacunas  into  irregular  channels 
or  primary  marrow  spaces.  Upon  the  spicules  of  calcified  cartilage 
matrix,  osteoblasts  arrange  themselves  and  begin  to  lay  down 
lamellae  of  bone.  These  changes  progress  from  the  center  in  both 


Hyaline 

cartilage 


Area  of 
calcification 


Capsules  containing 
"many  cartilage  cells 


FIG.  188. — From  a  longitudinal  section  of  a  finger  of  a  three-and-a-half-months 
human  embryo.  Two-thirds  of  the  second  phalanx  is  represented.  At  X  a  periosteal 
bud  is  to  be  seen.  (85  X)  (Szymonowicz.) 


directions,  and  all  stages,  from  the  typical  hyaline  cartilage  to  the 
formation  of  bone,  may  be  seen  in  one  section.  These  stages  are 
illustrated  by  Figs.  188,  189,  and  190. 

From  now  on  the  bone  grows  by  progressive  transformation  of 
cartilage  and  by  the  growth  of  bone  under  the  periosteum,  which 
will  be  considered  under  Bone  Growth. 


218 


BONE  FORMATION  AND  GROWTH 


Endomembranous  Bone  Formation. — The  bones  which  are  not 
preformed  in  cartilage  are  formed  directly  from  fibrous  tissue. 
This  is  well  illustrated  in  the  mandible.  In  the  region  of  Meckel's 
cartilage  and  between  it  and  the  developing  tooth  germs  the  mesen- 
chyme  begins  to  show  signs  of  specialization.  Delicate  fibers  appear 
in  the  intercellular  substance.  Along  these  the  connective-tissue 
cells  arrange  themselves,  and,  taking  on  the  form  of  osteoblasts, 
begin  to  lay  down  bone  lamellae  (Fig.  191).  These  stretch  out 


Cartilage  cell^.  | 


Periosteum—  m 


PericJiondral.. 

bone 


FIG.  189. — The  place  marked  X  in  the  preceding  figure  with  stronger  magnification. 
(185  X)     (Szymonowicz.) 


through  the  mesenchyme,  forming  a  network  of  delicate  spicules, 
until  they  surround  Meckel's  cartilage,  and  grow  up  to  the  buccal 
and  the  lingual  of  the  tooth  germs.  As  soon  as  this  network  of 
bone  lamellae,  containing  embryonal  connective  tissue  in  its  pri- 
mary marrow  spaces,  begins  to  take  on  definite  form,  there  is  a 
specialization  of  the  mesenchyme  surrounding  it,  developing  into 
fibrous  tissue  which  becomes  a  periosteum.  From  this  time  onward 
the  formation  of  bone  progresses,  as  will  be  described  under  the 
growth  of  bone. 


BONE  GROWTH 


219 


Bone  Growth. — If  sections  are  cut  transversely  through  the  shaft 
of  a  long  bone  from  a  fetus,  the  surface  will  be  found  to  be  covered 
by  a  well-formed  periosteum,  which  is  actively  laying  down  layers 
of  subperiosteal  bone.  The  central  portion  of  the  bone  is  made 
up  of  a  network  of  spicules  surrounding  primary  marrow  spaces, 


Periosteum 


Enlarged 
cartilage 

-elh 


Endochondral 
bone 


Periuxteal  hud 


Blood-vessels 
filled  with 


I    Perichoudral 
bone 


Calcified 
ca  rtilage 


FIG.  190.- — From  longitudinal  section  of  a  finger  of  a  four  months  embryo.    Only  the 
diaphysis  of  the  second  phalanx  is  represented.     (85  X)     (Szymonowicz.) 

there  being  no  true  marrow  cavity.  The  formation  of  the  subperios- 
teal layers  does  not  progress  at  a  uniform  rate  at  all  points  on  the 
circumference,  but  they  are  piled  up  at  certain  points  forming 
longitudinal  ridges  with  grooves  between  them.  These  grooves 
become  arched  across,  enclosing  part  of  the  connective  tissue  of 
the  inner  layer  of  the  periosteum,  and  contain  bloodvessels  and 


220 


BONE  FORMATION  AND  GROWTH 


nerves.  Soon  after  these  spaces  are  enclosed  absorptions  begin  in 
their  walls,  destroying  a  large  part  of  the  subperiosteal  lamellse 
and  forming  primary  marrow  spaces.  As  soon  as  these  spaces 
have  reached  a  certain  size  the  absorptions  stop,  and  osteoblasts 
appear  upon  the  wall  of  the  space  and  begin  to  lay  down  lamellae 


Osteo L 

V-1 * 


FIG.  191. — Section  through  the  lower  jaw  of  an  embryo  sheep  (decalcified  with 
picric  acid).  At  a  and  immediately  below  are  seen  the  fibers  of  a  primitive  marrow 
cavity  lying  close  together  and  engaged  in  the  formation  of  the  ground  substance 
of  the  bone,  while  the  cells  of  the  marrow  cavity,  with  their  processes,  arrange 
themselves  on  either  side  of  the  newly  formed  lamellae  and  functionate  as  osteoblasts. 
(Bohm,  Davidoff,  Huber.)  (300  X) 


upon  its  circumference,  until  an  Haversian  system  has  been  produced 
with  an  Haversian  canal  at  its  center.  In  this  way  the  bone  increases 
in  diameter,  and  this  process  continues  until  a  considerable  thickness 
of  Ilaversian  system  bone  is  formed.  In  all  bone  growth  there  is 
the  alternation  of  formation,  destruction,  and  rebuilding,  and  it 
must  be  remembered  that  this  continues  as  long  as  the  bone  func- 
tions as  an  organ  of  support.  As  the  shaft  becomes  larger  the 


GROWTH  OF  MEMBRANE  BONES  221 

primary  marrow  spaces  at  the  center  are  enlarged  by  the  absorp- 
tion, and  a  few  lamellae  are  laid  down  again  upon  their  walls,  until 
finally  in  the  central  portion  of  the  shaft  the  true  marrow  cavity 
is  formed.  As  the  thickness  of  Haversian  system  bone  becomes 
greater,  absorptions  occur  in  the  Haversian  canals,  cutting  out 
large,  irregular  channels,  around  which  a  few  lamellae  are  laid  down, 
and  so  the  Haversian  system  bone  becomes  converted  into  cancel- 
lous  bone  and  is  opened  into  the  marrow  cavity  as  it  grows  larger. 

Growth  of  Membrane  Bones. — The  growth  of  the  membrane  bone 
progresses  in  a  very  similar  way.  As  soon  as  the  periosteum  is 
formed  subperiosteal  bone  is  laid  down  and  converted  into  Haver- 
sian system  bone,  forming  the  compact  plate  of  the  surface,  leav- 
ing the  cancellous  portion  first  formed  at  the  center.  When  a 
certain  thickness  of  compact  bone  has  been  formed,  absorptions 
occur  in  the  Haversian  canals,  converting  the  deeper  portions  into 
cancellous  bone.  This  process  may  be  reversed.  Absorptions  may 
occur  under  the  periosteum,  cutting  deeply  into  the  Haversian 
system  bone,  and  then  a  few  subperiosteal  layers  laid  down 
upon  it.  When  this  occurs  lamellae  are  laid  down  around  the 
marrow  spaces,  converting  the  cancellous  bone  into  Haversian 
system  bone  to  maintain  the  required  strength.  In  this  way  the 
bones  are  moulded  into  shape,  adapting  them  to  the  mechanical 
conditions  to  which  they  are  subjected.  There  is  an  oscillation 
between  formation  and  destruction,  by  which  the  balance  adapted 
to  the  mechanical  conditions  is  maintained.  It  has  often  been  noted 
that  bones  are  never  allowed  to  become  more  bulky  than  is  neces- 
sary to  perform  their  function. 


CHAPTER  XVIII. 
PERIOSTEUM.1 

Definition. — The  periosteum  is  the  formative  and  protective 
membrane  which  covers  the  outer  surface  of  the  bone.  All  perios- 
teum has  certain  structural  characteristics  in  common,  but  because 
of  structural  differences  two  classes  are  recognized — attached  and 
unattached — each  of  which  may  be  simple  or  complex.  Perios- 
teum may  thus  be  classified  as  follows : 

1.  Unattached  simple. 

2.  Unattached  complex. 

3.  Attached  simple. 

4.  Attached  complex. 

Function  of  Periosteum. — The  importance  to  the  dentist  of  a 
knowledge  of  the  structure  and  function  of  the  periosteum  can 
scarcely  be  exaggerated.  It  has  been  the  knowledge  of  this  tissue 
and  its  function  that  has  led  to  all  the  advancement  in  bone  surgery 
of  modern  time.  Repair  and  regeneration  of  bone  is  largely  accom- 
plished through  its  agency. 

The  periosteum  forms  the  immediate  covering  of  all  the  bones 
and  is  continuous  over  their  entire  surface  except  the  portion  covered 
by  cartilage.  Each  bone  therefore  has  a  periosteum  of  its  own 
which  does  not  continue  around  the  articulation  to  the  bones  with 
which  it  joins.  Bones  that  are  united  by  suture  are,  however, 
covered  by  a  common  periosteum.  If  the  flesh  and  overlying 
tissues  are  carefully  removed  from  a  long  bone,  the  periosteum  will 
be  seen  as  a  smooth  white,  lustrous  membrane,  having  much  the 
same  appearance  of  a  tendon  on  most  of  its  surface.  But  at  some 
places  which  correspond  to  the  positions  where  muscles  or  fascia 

1  In  the  presentation  of  this  chapter  it  is  impossible  adequately  to  express  my 
indebtedness  to  Dr.  G.  V.  Black.  Almost  all  of  the  illustrations  are  taken  from  The 
Periosteum  and  Peridental  Membrane,  published  by  him  in  1887.  I  have  always 
felt  that  this  book  had  never  received  the  attention  it  deserves.  Only  one  thousand 
copies  of  it  were  printed,  and  they  were  not  sold  until  the  orthodontists  exhausted 
the  edition.  The  book  is  now  entirely  out  of  print  and  is  very  difficult  to  obtain.  I 
have  studied  this  book  for  years  and  have  repeated  almost  all  of  the  work  described 
in  it,  but  I  have  felt  that  it  was  impossible  for  text-book  purposes  to  improve  upon 
the  illustrations. 
(222) 


FUNCTION  OF  PERIOSTEUM  223 

were  attached  it  appears  ragged  and  dull,  for  the  tissues  had  to 
be  cut  to  separate  them  from  the  outer  layer  of  the  periosteum, 
to  which  they  were  firmly  adherent.  In  all  other  places  the  tissues 
separate  easily  in  dissection ;  in  fact,  are  not  attached  at  all,  except 
by  the  lightest  of  areolar  tissue,  which  is  very  easily  broken,  and 
the  tissues  may  be  separated  from  the  surface  of  the  membrane 
with  the  finger  or  the  handle  of  a  scalpel.  Now,  if  the  periosteum 
is  slit  along  a  smooth  surface  with  the  scalpel  and  the  handle 
inserted  between  the  bone  and  the  membrane,  it  will  be  found 
to  separate  readily  from  the  bone  over  most  of  its  surface.  If  the 
process  is  watched  closely,  little  strings  will  be  seen  apparently 
running  from  the  periosteum  to  the  bone,  and  being  broken  as  they 
are  separated.  These  are  mostly  small  bloodvessels  which  are 
running  into  canals  in  the  bone.  In  this  process  the  periosteum 
seems  like  a  closely  adapted  sac  or  elastic  glove,  clothing  the  sur- 
face of  the  bone,  as  if  surrounding  it  in  a  fibrous  bag.  If  the  sepa- 
ration of  the  periosteum  from  the  bone  is  continued,  it  will  be 
found  that  it  does  not  separate  as  easily  in  all  places.  As  the 
articular  ends  are  approached  it  becomes  suddenly  fastened  to  the 
underlying  bone,  and  the  blade  of  the  knife  must  be  used.  The 
periosteum  now  appears  as  a  very  thin,  tough,  and  inelastic  mem- 
brane, that  is  torn  with  difficulty,  but  it  is  so  thin  that  it  is  difficult 
now  to  separate  it  from  the  bone  without  cutting  it  through.  When 
this  point  of  attachment  is  reached  it  seems  that  the  periosteum  is 
sinking  into  the  substance  of  the  bone,  and  from  the  examination 
of  its  structure  it  is  found  that  this  is  practically  what  has  happened. 

Comparing  the  periosteum  to  a  sac  surrounding  the  bone,  it  is 
found  sewed  firmly  down  at  the  margin  of  the  cartilage  around 
the  articular  ends.  Besides  the  attachment  around  the  cartilage, 
the  periosteum  will  be  found  adherent  in  the  following  positions: 
^Yhere  muscles  or  fascia  are  attached  to  the  outer  layer  of  the 
periosteum;  where  it  approaches  the  insertion  of  tendons  or  liga- 
ments; and  where  the  skin  or  mucous  membrane  seem  attached  to 
the  underlying  bone,  as  around  the  auditory  meatus,  the  gums, 
mucous  membrane  of  the  nose,  etc.  In  all  such  positions  the 
periosteum  is  firmly  attached  to  the  bone— in  fact,  becomes  a  part 
of  it — and  through  this  medium  the  connections  between  muscles, 
fascia,  etc.,  and  the  framework  of  the  skeleton  is  accomplished. 

This  feature  of  the  anatomy  of  the  periosteum  has  never  been 
studied  in  the  detail  it  deserves,  especially  by  the  dentist.  It  is 
of  the  greatest  importance  in  the  management  of  the  diseases  of 


224  PERIOSTEUM 

bone,  especially  those  involving  the  formation  of  pus,  for  these 
lines  of  attachment  determine  the  direction  in  which  the  pus  will 
proceed  along  the  surface  of  the  bone.  When  pus  generated  within 
the  bone  reaches  the  surface,  it  will  lift  an  unattached  periosteum 
and  run  along  the  surface  until  it  reaches  a  line  of  attachment. 
Here  it  can  penetrate  the  periosteum  more  easily  than  it  can  sepa- 
rate it  from  the  bone.  When  a  line  of  attachment  is  reached,  there- 
fore, the  direction  of  the  burrowing  is  determined  by  the  attached 
areas.  The  pus  penetrates  the  periosteum  more  easily  than  it 
separates  its  attachments  from  the  bone,  but  it  lifts  the  unattached 
periosteum  so  easily  that  it  will  often  run  along  a  line  of  attach- 
ment for  a  long  distance. 

These  factors  often  become  of  great  importance  in  determining 
the  position  in  which  alveolar  abscesses  will  point.  For  instance, 
if  an  abscess  from  a  bicuspid  root,  or  the  mesial  root  of  a  molar, 
reaches  the  surface  of  the  bone  above  the  attachment  of  the  buc- 
cinator, it  cannot  penetrate  its  attachment  and  pass  downward 
to  open  on  the  gum,  but  may  run  out  over  the  surface  of  the  muscle 
and  open  on  the  cheek,  producing  the  crow's  foot  scar  so  often  seen. 
An  abscess  from  an  upper  cuspid  may  reach  the  surface  of  the  bone 
in  the  canine  fossa  between  the  attachments  of  the  nasalis  and 
caninus,  and  lift  the  periosteum  extending  upward,  and  open  at 
the  inner  canthus  of  the  eye  between  the  orbicularis  and  the  angular 
head  of  the  quadratus  labii  superioris.  If  these  abscesses  had  been 
reached  with  a  lance,  through  the  mucous  membrane,  at  the  proper 
time,  a  disfiguring  scar  \vould  have  been  avoided.  Accurate  knowl- 
edge of  the  attached  layers  of  the  periosteum  would  have  made  it 
certain  that  they  could  never  point  in  the  mouth  cavity  without 
assistance. 

Layers  of  the  Periosteum. — Periosteum  is  always  composed  of 
two  distinct  layers: 

1.  An  outer  or  fibrous  layer,  which  is  essentially  protective  and 
to  which  muscles  and  fascite  are  attached.     This  may  be  either 
simple  or  complex. 

2.  An  inner  or  osteogenetic  layer  which  is  essentially  the  vital 
functioning  layer,  and  is,  as  its  name  indicates,  concerned  with 
the  formation  of  bone.    This  may  be  either  simple  or  complex. 

The  Structural  Elements. — The  periosteum  is  composed  of  the 
following  structural  elements: 

1.  White  fibers  in  coarse  bundles  (in  the  outer  layer). 

2.  White  fibers  in  very  fine  bundles  (in  the  inner  layer). 


SIMPLE   UNATTACHED  PERIOSTEUM 


225 


3.  Elastic  fibers. 

4.  The  penetrating  fibers,  or  white  fibers  of  the  periosteum,  that 
in  the  growth  of  bone  are  included  in  its  substance. 

5.  Embryonal  connective-tissue  cells. 

6.  Osteoblasts  or  bone-forming  cells. 

7.  Osteoclasts  or  bone-absorbing  cells. 

Unattached  Periosteum. — In  the  unattached  periosteum  the  inner 
layer  is  always  simple,  and  the  outer  layer  may  be  either  simple 


' 


FIG.  192. — Non-attached  periosteum  from  the  shaft  of  the  femur  of  the  kitten: 
B,  bone;  O,  layer  of  osteoblasts.  In  the  central  portion  of  the  figure  they  have  been 
pulled  slightly  away  from  the  bone,  displaying  the  processes  to  advantage.  It  will 
be  observed  that  the  fibers  of  the  periosteum  do  not  enter  the  bone,  a,  inner  layer 
of  fine  white  fibrous  tissue  (osteogenetic  layer)  showing  the  nuclei  of  the  fibroblasts 
and  a  number  of  developing  connective-tissue  cells,  which  probably  become  osteo- 
blasts; c,  outer  layer,  or  coarse  fibrous  layer,  in  which  fusiform  fibroblasts  are  also 
rendered  apparent  by  double  staining  with  hematoxylin  and  carmine;  d,  some 
remains  of  the  reticular  tissue  connecting  the  superimposed  tissue  with  the  periosteum, 
(y'o  immersion.)  (Black.) 


or  complex,  depending  apparently  upon  the  requirements  of  pro- 
tection. In  general,  the  more  exposed  the  position  the  thicker  is 
the  layer,  and  the  larger  and  stronger  the  bundles  of  fibers  of  which 
it  is  composed. 

Simple  Unattached  Periosteum. — Where  the  periosteum  is  covered 
by  a  thick  layer  of  muscles  which  are  not  attached  to  it,  as  in  the 
thigh,  the  thinnest  and  simplest  form  of  periosteum  is  found.  An 
illustration,  drawn  by  Dr.  Black,  of  the  periosteum  from  the  femur 
of  a  kitten  will  illustrate  its  structure  (Fig.  192).  The  outer  layer 
15 


226  PERIOSTEUM 

is  composed  chiefly  of  bundles  of  white  fibers,  most  of  which  run 
in  a  direction  parallel  with  the  long  axis  of  the  bone.  The  bundles 
are  comparatively  small  and  much  flattened,  so  as  to  be  quite 
ribbon-like.  The  inner  layer  contains  a  much  greater  number  of 
cells  lying  among  extremely  delicate  fibers.  In  its  outer  portion 
many  of  the  cells  are  embryonal  in  character.  In  contact  with 
the  surface  of  the  bone  is  a  continuous  layer  of  osteoblasts  which 
are  building  subperiosteal  bone  in  the  young  animal,  processes  of 


FIG.  193. — Periosteum  from  the  shaft  of  the  tibia  of  the  pig,  lengthwise  section, 
showing  the  complex  arrangement  of  fibers  in  the  coarse  or  outer  fibrous  layer  that 
sometimes  occurs  under  muscles  that  perform  sliding  movements  upon  it:  B,  bone; 
O,  layer  of  osteoblasts.  The  tissue  has  been  pulled  slightly  away  from  the  bone  in 
mounting  the  section,  and  part  of  the  osteoblasts  have  clung  to  the  bone,  some 
have  clung  to  the  tissues,  while  others  are  suspended  midway,  their  processes  cling- 
ing to  each,  a,  layer  of  fine  fibers;  inner  or  osteogenetic  layer  of  the  periosteum; 
b,  first  lamina  of  the  coarse  or  outer  fibrous  layer,  the  fibers  of  which  are,  in  this  case, 
circumferential,  exposing  the  cut  ends.  It  will  be  observed  that  there  are  ten  lamina 
in  the  make  up  of  the  outer  layer,  the  lengthwise  and  circumferential  fibers  alternating. 
The  ones  marked/ and  i  are  very  delicate  ribbon-like  forms,  which  have  shifted  from 
their  normal  position  in  the  mounting  of  the  section,  so  as  to  present  their  sides  to 
view  instead  of  their  ends,  thus  displaying  their  structure  to  advantage.  The  illus- 
tration shows  how  readily  separable  these  lamina  are.  I,  reticular  tissue,  (j., 
immersion.)  (Black.) 


their  cytoplasm  extending  into  the  canaliculi  of  the  matrix  which 
they  have  formed.  At  one  point  in  the  illustration  the  osteoblasts 
are  pulled  off  from  the  surface  of  the  bone  and  show  these  processes 
stretched  out  of  the  canaliculi. 

Complex  Unattached  Periosteum. — In  some  places,  especially 
where  muscles  or  tendons  perform  sliding  movements  over  an 
unattached  periosteum,  the  outer  layer,  instead  of  being  simple, 
may  be  very  complex.  This  is  illustrated  in  Dr.  Black's  drawing 


SIMPLE  ATTACHED  PERIOSTEUM 


227 


(Fig.  193),  from  the  periosteum  of  the  tibia  of  a  young  pig.  In 
this  instance  the  outer  layer  is  composed  of  very  much  flattened 
bundles  of  white  fibers,  arranged  alternately  longitudinally  and 
circularly.  Ten  layers  may  be  counted  in  the  section.  The  inner 
layer  is  of  the  same  character  as  in  a  simple  specimen. 

Attached  Periosteum. — The  attached  periosteum  differs  from  the 
unattached  by  having  the  fibers  of  the  inner  layer  arranged  in 
bundles,  around  which  the  bone  matrix  is  deposited  by  the  osteo- 
blasts,  embedding  them  in  the  substance  of  the  matrix  and  calcify- 


FIG.  194. — Simple  attached  periosteum:  a,  bone;  b,  osteo blasts;  c,  fibers  of  the 
inner  layer;  D,  bloodvessels  of  the  inner  layer;  E,  outer  layer;  F,  muscle  fibers  at- 
tached to  outer  layer.  (Black.) 


ing  them  with  it.  These  fibers  constitute  the  penetrating  fibers. 
They  were  first  described  by  Sharpey,  and  have  been  called  Shar- 
pey's  fibers.  He,  however,  apparently  did  not  understand  their 
importance  or  manner  of  formation.  The  fibers  of  the  inner  layer 
are  built  into  the  substance  of  the  bone  in  this  way  wherever  tissues 
are  attached  to  the  outer  layer  of  the  periosteum. 

Simple  Attached  Periosteum. — Where  the  pull  of  tissues  attached 
to  the  outer  layer  of  the  periosteum  is  in  one  direction,  the  fibers 
of  the  inner  layer  are  inclined  in  the  same  direction  (Figs.  194  and 


228 


PERIOSTEUM 


195).     As  the  surface  of  the  bone  is  approached  the  fibers  are 
gathered  into  strong  bundles  to  be  inserted  in  the  bone,  the  osteo- 


FIG.  195. — A  photomicrograph  of  an  attached  periosteum  similar  to  Fig.  194.    From 
the  alveolar  process  of  a  sheep.     (About  80  X) 


FIG.  196. — Attached  periosteum  from  beneath  the  attachment  of  the  muscles  of 
the  lower  lip  of  the  sheep:  A,  bone;  B,  osteoblasts,  with  the  fibers  emerging  from 
the  bone  between  them;  C,  inner  layer  with  fibers  decussating  and  joining  the 
inner  side  of  the  coarse  fibrous  layer  in  opposite  directions  (this  is  rather  an  unusual 
form  of  this  layer  of  the  periosteum);  D,  coarse,  fibrous  layer;  E,  attachment  of 
muscular  fibers.  (Black.) 


blasts  covering  the  surface  of  the  bone  everywhere  between  the 
fibers.     The  outer  and  inner  layers  are  united  by  the  interlacing 


COMPLEX  ATTACHED  PERIOSTEUM  229 

of  their  fibers.    At  the  junction  of  the  outer  and  the  inner  layers 
many  bloodvessels  are  seen. 

Complex  Attached  Periosteum. — Where  the  pull  upon  the  outer 
layer  is  in  many  directions,  the  fibers  of  the  inner  layer,  after  emerg- 
ing from  the  bone,  break  up  into  smaller  bundles  and  anastomose 
in  all  directions,  arching  around  to  interlace  with  the  fibers  of  the 
outer  layer,  and  in  this  way  they  sustain  force  in  all  directions 
(Fig.  196).  This  is  illustrated  in  Dr.  Black's  drawing  of  a  section 
of  attached  periosteum  from  beneath  the  attachment  of  the  muscles 
of  the  lower  lip  of  a  sheep. 


CHAPTER  XIX. 
THE  ATTACHMENT  OF  THE  TEETH. 

THAT  the  teeth  are  not  a  part  of  the  osseous  system,  but  are 
appendages  of  the  skin,  supported  in  man  by  a  special  development 
of  bone  forming  the  alveolar  ridges  of  the  maxillary  bones,  is  as 
well  established  as  any  fact  concerning  human  dentition.  The 
work  of  Oscar  Hertwig,  published  in  1874,  established  very  clearly 
the  homology  existing  between  the  teeth  and  the  dermal  or  placoid 
scales  of  the  ganoid,  silurioid,  and  dipnoan  fishes,  both  as  to  simi- 
larity of  structure  and  development. 

Much  has  been  written  descriptive  of  the  teeth  of  various  animals, 
their  modifications  of  form,  and  attachment  to  adapt  them  to 
modifications  of  function,  and  various  classifications  of  the  means 
of  attachment  have  been  made.  Of  these,  perhaps  the  best  and 
most  logical  is  given  by  Charles  Tomes  in  his  Dental  Anatomy, 
describing  four  forms  of  attachment:  (1)  By  fibrous  membrane; 
(2)  by  hinge-joint;  (3)  by  ankylosis;  (4)  by  insertion  in  a  socket 
or  gomphosis. 

These  various  forms  of  attachment  will  be  taken  up,  and,  if 
possible,  the  comparison  between  them  and  the  evolution  of  the 
more  complicated  forms  from  the  simpler  will  be  shown.  The  study 
must  begin  with  an  examination  of  the  structure  and  attachment 
of  the  placoid  scales  and  the  simplest  form  of  tooth,  as  illustrated 
in  the  shark. 

Structure  of  Dermal  Scales. — The  dermal  scales  are  composed 
of  a  conical  cap  of  calcified  tissue  developed  from  within  outward, 
by  an  epithelial  organ,  and  corresponding  in  structure  to  the 
enamel.  This  cap  is  supported  upon  a  conical  papilla  of  calcified 
tissue  formed  from  without  inward,  and  corresponding  to  dentin. 
In  the  outer  layer  the  arrangement  of  the  fine  tubules  through  the 
calcified  matrix  correspond  very  closely  to  human  dentin,  but  in 
the  inner  portions  it  is  to  be  understood  only  by  considering  the 
formation  of  the  dentin  as  progressing  irregularly  over  the  surface 
of  the  pulp  and  so  dividing  the  pulp  tissue  into  portions  enclosed 
in  large  canals,  from  which  the  fine  tubules  radiate.  The  base  of 
(230) 


ATTACHMENT  BY  HINGE  JOINT 


231 


this  partially  calcified  papilla  has  a  calcified  connective  tissue 
built  on  to  it  by  the  derma  or  connective-tissue  layer  of  the  skin, 
which  corresponds  to  cementum  forming  the  basal  plate,  spreading 
out  more  or  less  in  the  connective-tissue  layer  of  the  skin,  and 
into  which  the  fibers  of  this  layer  are  built,  so  attaching  the  denticle 
or  dermal  scale  to  the  deep  layer  of  the  coreum.  This  tissue  very 
exactly  resembles  cementum.  It  is  formed  on  the  dentin  as  the 
cementum  of  a  human  tooth  is,  and  shows  the  connective-tissue 
fibers  embedded  in  it.  In  the  ganoids  the  basal  plates  of  adjoining 
scales  unite,  forming  the  armor  plates  of  such  fish  as  the  sturgeon 
and  gar-pike,  and  the  dentical  remains  projecting  from  the  surface 
of  the  plates. 


FIG.  197. — Showing  additions  of  bone  of  attachment  to  the  bone  of  the  jaw. 

(Tomes.) 

Attachment  by  Fibrous  Membrane. — In  the  simplest  teeth,  as  of 
the  shark  (Lamna  cornubica,  Fig.  3),  which  are  typical  dermal 
scales,  there  is  an  exactly  similar  method  of  attachment,  which 
may  be  taken  as  the  simplest  and  most  rudimentary,  or  attachment 
in  a  fibrous  membrane.  That  is,  there  is  no  development  or  modi- 
fication of  the  arch  of  the  jaw,  and  the  teeth  have  no  direct  attach- 
ment to  the  bone;  in  fact  (Fig.  197),  the  jaws  themselves  are  chiefly 
cartilage. 

Attachment  by  Hinge  Joint. — The  formation  of  the  hinge  attach- 
ment as  illustrated  in  many  of  the  fishes  (Fig.  198),  may 
be  understood  as  a  modification  of  the  attachment  in  a  fibrous 


232 


THE  ATTACHMENT  OF  THE  TEETH 


membrane  in  a  more  highly  specialized  creature.  These  hinged 
teeth  are  found  in  many  fishes  and  in  the  poison  fangs  of  snakes. 
The  jaws  are  calcified,  and  the  basal  plate  or  cementum  may  be 
considered  as  confined  to,  or  specially  developed  on,  one  side  of 
the  dentin  papilla,  which  is  also  more  highly  developed,  especially 
in  snakes.  This  cementum  is  built  and  calcified  around  the  fibers 
of  the  fibrous  tissue  which  pass  directly  to  the  bone  of  the  jaw 


FIG.  198. — Attachment  by  hinge  joint.  Tooth  of  a  hake:  a,  vasodentin;  b,  pulp; 
c,  elastic  hinge;  d,  buttress  to  receive  /,  formed  out  of  bone  of  attachment;  e,  bone 
of  jaw;  /,  thickened  base  of  tooth;  g,  enamel  tip.  (Tomes.) 

at  that  point.  This  bone  is  to  be  regarded  as  an  addition  to  the 
jaw  specially  developed  for  each  tooth.  Thus,  there  is  not  only  a 
modification  in  the  arrangement  of  the  cementum,  but  a  develop- 
ment of  bone  for  attachment  of  the  tooth.  The  bloodvessels  pass 
through  the  fibers  of  the  hinge  to  the  pulp,  and  are  not  affected  by 
the  motion  of  the  tooth  on  the  hinge;  in  fact,  the  pulp  seems  to  be 
attached  to  the  hinge.  There  are  many  complications  of  this 


ATTACHMENT  BY  ANKYLOSIS  233 

method  of  attachment,  but  this  may  be  taken  as  the  type  and  the 
manner  of  its  modification  from  the  rudimentary  conditions.  The 
distinction,  in  this  form  of  attachment,  from  the  dermal  scale  con- 
sists in  a  modification  of  the  arrangement  of  the  cementum  of  the 
basal  plate  and  a  development  of  bone  from  the  jaw  to  attach 
fibers  which  pass  directly  from  cementum  to  bone.  It  should  also 
be  said  that  there  are  developments  in  the  hinge  teeth  related  to 
the  third  form  of  attachment,  namely,  ankylosis,  which  cannot 
be  understood  until  this  form  is  studied. 

Attachment  by  Ankylosis. — The  third  form  of  attachment,  anky- 
losis (Fig.  199),  or  direct  calcified  union  with  the  bone  of  the  jaw, 
cannot  be  understood  without  a  careful  study  of  the  nature  and 
formation  of  the  dentin  in  these  rudimentary  teeth.  It  is  evident, 
from  a  study  of  the  dentin  of  the  dermal  scales,  that  compared 
with  human  dentin,  the  tissue  is  rudimentary  and  not  differentiated 
from  other  similar  connective  tissues.  The  tubules  are  compara- 
tively very  irregular,  and  resemble  strikingly  the  tubules  found  in 
the  secondary  dentin  formed  by  a  degenerating  pulp.  The  odonto- 
blasts,  or  dentin-forming  cells,  are  not  like  the  highly  specialized 
cells  which  form  the  primary  human  dentin,  but  resemble  very 
closely  simple  spindle-shaped  connective-tissue  cells.  The  nucleus 
is  larger  and  oval  in  form,  and  the  protoplasm  stretches  off  from 
it  in  one  direction  into  a  fibril  instead  of  in  two  directions  into  a 
spindle.  The  cells  are  much  smaller  than  human  odontoblasts  and 
nearer  the  size  of  ordinary  spindle  cells  of  the  human  pulp.  In 
fact,  they  look  more  like  specially  developed  spindle  cells  than 
odontoblasts.  The  formation  of  dentin  begins  on  the  surface,  at 
the  apex  of  a  cone-shaped  papilla  of  connective  tissue,  and  proceeds 
inward.  If  the  formation  continues  uniformly  over  the  surface 
of  the  papilla,  a  solid  layer  of  fine-tubuled  dentin  results;  but  it 
often  proceeds  irregularly,  apparently  having  special  reference 
to  the  neighborhood  of  bloodvessels,  so  that  irregular  projections 
of  dentin  are  found  on  its  inner  surface,  dividing  the  pulp  more  or 
less  into  portions  enclosed  in  larger  channels  or  tubes.  These  may 
be  very  regular  in  arrangement  and  form  around  bloodvessels 
loops  embedding  the  bloodvessel  in  the  calcified  tissue,  producing 
what  has  been  called  vaso  or  vascular  dentin;  but  the  formation 
is  still  from  the  surface  of  the  pulp  until  it  is  obliterated,  except 
for  what  remains  in  the  larger  canals.  As  distinguished  from  this 
formation  of  dentin  we  find  in  the  body  of  the  dental  papilla  of 
many  fishes  the  formation  of  spicules  of  calcified  tissue,  which 


234  THE  ATTACHMENT  OF  THE  TEETH 

resemble  neither  dentin  nor  typical  bone,  shooting  down  through 
the  substance  of  the  pulp.  They  are  more  to  be  compared  with 
the  first  formation  of  bone  in  membranes,  or  in  the  embryonal 
connective  tissue  of  the  body  of  the  human  jaw,  which  is  afterward 


FIG.  199. — Tooth  of  scarus,  showing  attachment  by  ankylosis:  1,  vertical  section 
of  five  pharyngeal  teeth  of  Scarus  muricatus;  2,  section  of  a  single  tooth  magnified: 
a,  osteodentin;  b,  dentin;  c,  enamel;  d,  cementum;  3,  termination  of  a  single 
dentinal  tubule.  (Owen.) 

removed  by  absorption  and  replaced  by  true  Haversian  system 
bone.  These  calcifications  contain  lacunae,  and  have  tubules  or 
canaliculi  running  through  them,  and  so,  as  Tomes  says,  are  inter- 


ATTACHMENT  BY  IMPLANTATION  IN  SOCKET 


mediate  between  dentin  and  bone.  They  divide  the  pulp  into 
irregular  spaces,  and  interdigitate,  or  perhaps  actually  join,  the 
formation  of  dentin  which  has  been  progressing  from  the  surface 
of  the  pulp.  These  spicules  run  down  into  the  bone  of  the  jaw, 
forming  an  actual  calcified  attachment  for  the  tooth  with  the 
jaw;  but  in  this  view  of  it,  it  is  to  be  regarded  as  a  calcification  or 
rather  a  formation  of  bone  in  the  pulp  papilla  interlocking  with 
the  dentin.  In  some  of  the  fishes,  as  in  Scarus,  there  is  at  the  same 
time  the  remains  of  the  cementum  of  the  basal  plate  formed  on  the 
outside  of  the  dentin  around  the  base  of  the  cone.  Ankylosis  is 
confined  to  the  teeth  of  many  fishes,  and  may  be  stated  as  a  modi- 
fication from  the  dermal  scale,  resulting  in  the  reduction  or  loss  of 


FIG.  200. — A,  diagrams  of  tranverse  sections  through  the  jaws  of  reptiles  showing 
pleurodont  (a),  acrodont  (b),  and  thecodont  (c)  dentitions.  B.  a,  lower  jaw  of  Zootoca 
vivipara;  b,  of  anguis  fragilis.  (After  Leydig.)  Weidersheim,  Comparative 
Anatomy  of  Vertebrates.) 

the  basal  plate  and  an  ossification  of  the  pulp  continuing  through 
the  connective  tissue  at  the  base  of  the  pulp  to  the  body  of  the  jaw. 
Attachment  by  Implantation  in  Socket. — The  development  of  the 
fourth  form  of  attachment,  by  implantation  in  a  socket,  seems  to 
be  an  evolution  starting  from  the  same  point  but  proceeding  in 
a  different  direction  (Fig.  200).  It  is  associated  with  the  very 
great  increase  in  the  size  of  the  teeth  and  consequent  necessity  for 
a  stronger  attachment.  The  evolution  of  this  is  illustrated  in  the 
teeth  of  reptiles.  Weidersheim  classifies  the  teeth  of  reptiles  as 
(1)  resting  upon  a  ledge  on  the  lingual  side  of  the  jaw — pleurodont 
dentition;  (2)  resting  on  a  slight  ridge  around  them — acrodont 
dentition;  (3)  lodged  in  permanent  alveoli,  as  in  the  crocodile — 
thecodont  dentition.  These  three  classes  illustrate  three  stages  in 
the  development  of  the  socket  method  of  attachment. 


236  THE  ATTACHMENT  OF  THE  TEETH 

In  the  simplest  form  there  is  a  cone-shaped  tooth,  attached 
to  the  bone  around  its  base,  by  the  fibers  being  built  into  the 
cementum  and  bone.  There  is  little  modification  of  the  rudimentary 
form,  and  little  development  of  bone  for  its  attachment.  In  a 
higher  form  the  tooth  has  become  long  or  peg-shaped,  and  the  bone 
has  grown  up  around  a  portion  of  it  to  support  it;  but  it  is  attached 
to  the  bone  by  connective-tissue  fibers,  being  built  into  the  cemen- 
tum on  the  surface  of  the  tooth  and  into  the  bone  of  attachment  on 
the  jaw.  The  development  of  the  form  of  the  tooth  to  the  peg  from 
the  cone  may  be  understood  as  a  continuing  of  the  development  of 
odontoblasts,  and  the  formation  of  dentin  (which  always  begins 
at  the  apex  of  the  cone)  farther  and  farther  down  the  sides  of  the 
dental  papillae.  Then  the  formation  of  the  cementum,  which  begins 
around  the  base  of  the  cone  and  continues  down  on  the  outside  of 
the  calcified  dentin,  covering  its  outer  surface,  and  building  the 
connective-tissue  fibers  into  the  tooth.  The  development  of  bone 
accompanies,  or  rather  follows  that  of  the  tooth,  building  the 
other  ends  of  these  fibers  into  the  bone  which  is  developed  to 
support  the  tooth. 

Summary. — To  review  the  subject  matter  of  this  chapter,  all 
teeth  have  been  evolved  from  the  simple  placoid  scale.  In  the 
simplest  forms,  as  in  the  teeth  of  the  shark,  there  is  no  relation 
to  the  bone  whatever,  but  the  fibers  of  the  subcutaneous  tissue 
are  built  into  the  basal  plate  of  cementum.  As  the  tooth  becomes 
larger  and  demands  more  support,  there  is  added  to  the  bone  of 
the  jaw  that  which  Tomes  has  called  bone  of  attachment.  The 
osteoblasts  build  up  additions  to  the  jaw  which  surround  and 
embed  the  fibers,  so  that  the  fibers  which  were  originally  in  the 
subcutaneous  tissue  are  fastened  to  the  bone  at  one  end  and  to 
the  cementum  at  the  other.  The  evolutions  of  attachment  by 
hinge  joint  and  by  gomphosis  are  therefore  direct  evolutions  from 
the  simple  attachment  in  membrane.  The  form  of  ankylosis  is 
also  evolved  from  the  simplest  type,  but  in  this  case  the  bone  of 
attachment  is  associated  with  the  pulp,  and  the  formation  of  bone 
and  dentin  become  interlocked  and  united. 


CHAPTER  XX. 
THE  PERIDENTAL  MEMBRANE. 

IN  one  sense  the  peridental  membrane  may  be  considered  as  the 
most  important  tissue  to  the  dentist,  for  upon  it  the  usefulness 
of  the  teeth  and  their  comfort  to  the  individual  is  dependent.  It 
makes  no  difference  how  perfect  a  crown  may  be,  or  how  perfectly 
any  damage  which  may  have  occurred  to  it  may  have  been  restored, 
unless  the  peridental  membrane  is  in  a  healthy  and  fairly  normal 
condition,  the  tooth  will  be  useless,  and  the  individual  would  be 
much  more  comfortable  without  it. 

Definition. — The  peridental  membrane  may  be  defined  as  that 
tissue  which  fills  the  space  between  the  surface  of  the  root  and  the 
bony  wall  of  its  alveolus,  surrounds  the  root  occlusally  from  the 
border  of  the  alveolus,  and  supports  the  gingivse.  It  is  necessary 
to  emphasize  the  three  parts  of  the  definition.  The  peridental 
membrane  does  not  stop  at  the  border  of  the  bone,  but  continues 
to  surround  the  root  as  far  as  the  tissues  are  attached  to  it.  In 
general,  the  dental  profession  has  thought  of  the  peridental  mem- 
brane as  only  that  tissue  which  occupies  the  space  between  the 
root  and  the  wall  of  its  alveolus.  As  will  be  seen  from  a  study  of 
sections  later  (Figs.  203  and  204),  the  structure  of  the  tissue  sur- 
rounding the  root  between  the  gingival  line  and  the  border  of  the 
process  is  essentially  the  same  as  that  in  the  alveolus,  and  quite 
different  from  the  much  coarser  fibrous  mat  forming  the  submucous 
layer  of  the  gum  tissue.  The  peridental  membrane  also  extends 
into  the  free  margin  of  the  gum  and  is  the  means  of  its  support, 
holding  the  gingivse  close  to  the  surface  of  the  tooth  and  supporting 
them  in  the  interproximal  spaces.  The  importance  of  this  portion 
of  the  peridental  membrane  and  the  functions  which  it  performs 
have  been  strongly  emphasized  in  the  last  few  years,  in  their  rela- 
tion to  the  extensions  of  caries  and  the  beginnings  of  pyorrhea. 
Most  of  the  diseases  of  the  peridental  membrane  which  result  in 
the  final  loss  of  the  teeth  have  their  beginnings  in  this  portion. 

Nomenclature. — The  peridental  membrane  belongs  to  the  class 
of  fibrous  membranes  which  form  the  covering  of  organs,  the  cap- 

(237) 


238 


THE  PERIDENTAL  MEMBRANE 


sules  of  glands,  and  especially  those  membranes  which  cover  the 
organs  of  support.  Its  closest  relative  is  the  periosteum  in  the 
attached  portions,  with  which  it  has  many  points  of  structure 
in  common,  but  it  differs  from  the  periosteum  in  any  position  in 
important  respects.  It  has  often  been  called  the  alveodental 
periosteum,  but  this  name  implies  that  the  periosteum  is  folded 
down  into  the  alveolus  and  back  upon  the  surface  of  the  root, 
which  is  an  entirely  erroneous  conception  of  the  membrane.  This 
idea  would  imply  that  it  was  a  double  membrane  having  one  layer 
covering  the  bone  and  another  covering  the  root,  the  two  uniting 
in  the  middle  portions.  But  instead,  the  periosteum  must  be 


FIG.  201. — Drawing  to  show  the  arrangement  of  the  fibers  in  a  labiolingual  section 
through  an  incisor  of  a  kitten.     (Black.) 

considered  as  stopping  at  the  border  of  the  alveolus,1  and  being 
united  with  the  peridental  membrane  around  its  circumference. 
Many  writers  use  the  word  pericementum  in  place  of  peridental 
membrane.  The  author  prefers,  and  in  this  book  will  use,  the  term 
peridental  membrane,  though  the  two  are  synonymous. 

Divisions.— Purely  for  convenience  in  description,  the  peridental 
membrane  is  divided  into  three  portions:  The  gingival  portion, 
that  portion  of  the  membrane  which  surrounds  the  root  occlusally 

1  The  student  must  be  reminded  that  the  word  alveolus  means  a  hole,  and  the 
alveolar  process,  the  portion  of  the  bone  which  contains  the  holes.  In  dental  writing 
the  word  alveolus  has  often  been  incorrectly  used  in  place  of  process  or  alveolar 
process. 


PLATE  XIV 


Longitudinal  Section  of  Peridental  Membrane. 

Stained  \vith  hematoxylin  and  eosin.      Showing  border  of  alveolar  process. 


DIVISIONS 


239 


from  the  border  of  the  alveolar  process  and  supports  the  gingivse; 
the  alveolar  portion,  the  portion  of  the  membrane  from  the  border 


G  «' 


FIG.  202. — Diagram  of  the  fibers  of  the  peridental  membrane:  G,  gingival  portion; 
AL,  alveolar  portion;  Ap.,  apical  portion.  (From  a  photograph  of  a  section  from 
incisor  of  sheep.) 


240  THE  PER1DENTAL  MEMBRANE 

of  the  process  to  the  region  of  the  apex  of  the  root;  and  the  apical 
portion,  which  surrounds  the  apex  of  the  root  and  fills  the  apical 
space.  These  are  illustrated  in  the  diagram  (Figs.  201  and  202). 

The  Structural  Elements. — These  are:  (1)  White  connective- 
tissue  fibers;  (2)  fibroblasts;  (3)  cementoblasts;  (4)  osteoblasts;  (5) 
osteoclasts;  (6)  epithelial  structures  which  have  sometimes  been 
called  the  glands  of  the  peridental  membrane;  (7)  bloodvessels; 
(8)  nerves;  (9)  lymphatic  vessels. 

Functions. — The  peridental  membrane  performs  three  functions: 
(1)  A  physical  function — it  maintains  the  tooth  in  relation  to  the 
adjacent  hard  and  soft  tissues.  (2)  A  vital  function — the  formation 
of  bone  on  the  alveolar  wall  and  of  cementum  on  the  surface  of 
the  root.  (3)  A  sensory  function — the  sensation  of  touch  for  the 
tooth  being  exclusively  in  this  membrane. 

It  is  necessary  to  emphasize  the  two  parts  of  the  physical  func- 
tion; the  peridental  membrane  not  only  supports  the  teeth  in  their 
relation  to  the  bones  which  carry  them,  and  sustains  them  against 
the  forces  of  occlusion  and  mastication,  but  it  also  sustains  the 
soft  tissues  in  their  proper  relation  to  the  teeth.  The  second  part 
of  the  physical  function  is  fully  as  important  as  the  first,  and  the 
study  of  the  structure  of  the  tissue  related  to  it  and  the  adaptation 
of  the  form  of  the  gingivse  to  the  anatomic  form  of  the  teeth  and 
alveolar  process,  are  important  considerations  which  should  never 
be  lost  sight  of  in  the  making  of  artificial  crowns. 

Classes  of  Fibrous  Tissue. — The  fibrous  tissue  of  the  peridental 
membrane  is  entirely  of  the  white  variety,  but  may  be  divided 
into  two  classes.  The  principal  fibers  and  the  indifferent  or  inter- 
stitial tissue.  The  former  perform  the  physical  function  of  the 
membrane,  the  latter  simply  fill  in  spaces  between  the  bundles  of 
fibers  and  surround  *and  accompany  the  bloodvessels  and  the 
nerves. 

The  Principal  Fibers  of  the  Peridental  Membrane. — These  may 
be  defined  as  the  fibers  which,  springing  from  the  cementum,  are 
attached  at  their  other  extremities  to  the  connective  tissue  support- 
ing the  epithelium,  the  fibrous  mat  of  the  gum  tissue,  the  cementum 
of  the  approximating  tooth,  the  outer  layer  of  the  periosteum  at 
the  border  of  the  alveolar  process,  or  the  bone  of  the  alveolar  wall. 

Arrangement. — The  principal  fibers  literally  spring  from  the 
cementum,  the  cementoblasts  building  up  the  matrix  around  them 
and  then  calcifying  both  the  matrix  and  the  fibers,  in  this  way 
attaching  them  to  the  surface  of  the  root.  In  most  places  the  fibers 


ARRANGEMENT  241 

as  they  spring  from  the  cementum  appear  as  good-sized  bundles. 
A  short  distance  from  the  surface  of  the  root  they  may  break  up 
into  smaller  bundles  which  anastomose  and  interlace,  passing 
around  bloodvessels  and  other  fibers  in  their  course  and  being 
again  united  into  large  bundles  for  attachment  at  their  other 
extremity. 

To  arrive  at  an  understanding  of  the  arrangement  of  the  fibers 
of  the  peridental  membrane,  sections  must  be  cut  longitudinally, 
both  from  buccal  to  lingual  and  from  mesial  to  distal,  and  trans- 
versely through  all  portions  of  the  membrane.  It  therefore  requires 
the  study  of  many  sections  to  work  out  a  complete  conception. 
After  studying  them  out  completely  in  this  way  one  is  impressed 
with  the  beautiful  adaptation  of  their  arrangement  to  sustain  the 
tooth  against  all  the  forces  to  which  it  is  subjected,  and  to  support 
the  free  margin  of  the  gum,  so  that  it  will  lie  closely  against  the 
gingival  portion  of  the  enamel.  It  is  necessary,  however,  to  remind 
the  student  that  connective  tissues  are  formed  in  response  to 
mechanical  conditions  and  stimuli,  and  therefore  this  arrangement 
must  be  considered,  not  as  having  been  designed  to  sustain  the 
forces,  but  as  being  the  result  of  the  forces  to  be  sustained,  and 
therefore  beautifully  adapted  to  them. 

The  principal  fibers  of  the  peridental  membrane  are  naturally 
divided  into  a  number  of  groups  which  differ  in  their  arrangement 
and  function.  In  his  latest  book,  Special  Dental  Pathology,  Dr. 
Black  has  given  descriptive  names  to  these  groups.  Passing  from 
the  gingival  line  toward  the  apex  of  the  root  these  groups  are: 
(1)  The  free  gingival  group,  the  fibers  of  which  pass  from  the  cemen- 
tum occlusally  into  the  gingiva  to  support  it;  (2)  the  trans-septal 
group,  passing  from  tooth  to  tooth,  and  supporting  the  inter- 
proximal  gingivse;  (3)  the  alveolar  crest  group  passing  from  the 
cementum  to  the  outer  layer  of  the  periosteum  on  the  labial  and 
lingual  and  to  the  crest  of  the  alveolar  process  on  the  mesial  and 
distal;  (4)  the  horizontal  group  in  the  occlusal  third  of  the  alveolar 
portion  and  passing  at  right  angles  to  the  axis  of  the  tooth  from 
the  cementum  to  the  bone;  (5)  the  oblique  group  in  the  apical 
two-thirds  of  the  alveolar  portion  and  inclined  occlusally  as  they 
pass  from  cementum  to  bone;  (6)  the  apical  group,  the  group  of 
fibers  radiating  from  the  apex  of  the  root  to  the  bone  around  the 
apical  space. 

Beginning  at  the  gingival  line,  the  fibers  springing  from  the 
cementum  pass  out  at  a  short  distance  at  right  angles  to  its  surface 
16 


242  THE  PERIDENTAL  MEMBRANE 

and  then  bend  sharply  to  the  occlusal,  passing  up  into  the  gingivse 
and  uniting  with  the  fibrous  mat  which  supports  the  epithelium. 
These  are  much  more  strongly  marked  on  the  lingual  than  on  the 
labial  gingivse,  because  in  mastication  the  lingual  gingivse  receives 
more  pressure  of  food,  which  would  tend  to  crush  it  down  (the  free 
gingival  group).  A  little  deeper  the  fibers  springing  from  the 
cementum  on  the  labial  and  lingual  pass  out  at  right  angles  to 
the  cementum  and  are  lost  in  the  coarser  fibrous  mat  of  the  gum 
tissue.  The  distance  which  they  extend  before  being  lost  in  the 
coarser  fibers  is  always  greater  on  the  lingual  than  on  the  labial. 
On  the  proximal  sides  the  fibers  springing  from  the  cementum  at 
the  same  level,  branch  and  interlace,  passing  across  the  inter- 
proximal  space,  to  be  attached  to  the  cementum  of  the  approxi- 
mating tooth.  These  fibers  are  of  the  greatest  importance,  a? 
they  produce  the  basket  work  which  forms  the  supporting  frame- 
work for  the  interproximal  gingivse  (the  trans-septal  group).  A 
little  farther  apically  the  fibers  as  they  come  from  the  cementum 
are  inclined  apically.  A  short  distance  from  the  cementum  they 
unite  into  very  large  and  strong  bundles  which  join  with  the  fibers 
of  the  outer  layer  of  the  periosteum,  extending  over  the  labial 
and  lingual  border  of  the  alveolar  process  (the  alveolar  crest  group). 
On  the  proximal  sides  the  fibers  at  this  level  are  attached  to  the 
cementum  of  the  adjoining  tooth,  or  are  inclined  apically,  to  be 
inserted  in  the  bone  of  the  septum  (the  alveolar  crest  group) .  These 
large  bundles  form  a  distinct  layer,  which  has  been  called  the  dental 
ligament,  because  they  bind  the  teeth  together  across  the  septum 
and  attach  them  to  the  outer  layer  of  the  periosteum  on  the 
labial  and  lingual  borders  of  the  alveolar  process.  They  are  the 
only  fibers  which  hold  the  teeth  down  in  its  socket.  At  the  border 
of  the  alveolar  process,  and  in  the  occlusal  third  of  the  alveolar 
portion,  the  fibers  pass  directly  from  the  cementum  to  the  bone 
at  right  angles  to  the  axis  of  the  tooth  (the  horizontal  group).  In 
this  position  the  fibers  are  larger  and  stronger,  and  show  less  ten- 
dency to  break  up  into  smaller  bundles  in  their  course  than  in 
any  other  portion  of  the  membrane.  In  the  middle  and  apical 
thirds  of  the  alveolar  portion  the  fibers  are  inclined  occlusally  as 
they  pass  from  the  cementum  to  the  bone.  They  spring  from  the 
cementum  in  compact  bundles,  and  show  a  strong  tendency  to 
break  up  into  fan-shaped  fasciculi,  spreading  out  as  they  approach 
the  bone,  to  be  attached  over  a  larger  area  of  the  alveolar  wall. 
These  fibers  literally  swing  the  tooth  in  its  socket  and  support  it 


PLATE  XV 


'• 


Longitudinal  Section  of  Peridental  Membrane. 


ARRANGEMENT 


243 


against  the  forces  of  mastication  (the  oblique  group).  In  the  apical 
region  fibers  springing  from  the  cementum  pass  out  in  all  direc- 
tions, spreading  out  in  the  same  way,  to  be  inserted  into  the  bone 
forming  the  wall  of  the  apicals  pace  (the  apical  group) . 

If  force  is  exerted  against  the  lingual  surface  of  an  incisor,  the 
fibers  on  the  lingual  side  of  the  root  in  the  occlusal  third  will  sustain 
part  of  the  strain,  preventing  the  crown  from  moving  labially, 


FIG.  203. — Longitudinal  section  of  the  peridental  membrane  in  the  gingival  portion 
from  a  lamb   (the  labial  gingivus). 


and  at  the  same  time  the  fibers  on  the  labial  side  of  the  root  in  the 
apical  space  will  also  be  under  strain,  preventing  the  apex  of  the 
root  from  moving  lingually.  The  general  plan  of  arrangement 
which  has  been  described  is  illustrated  in  Dr.  Black's  diagram  made 
from  a  labiolingual  section  of  an  incisor  of  a  young  kitten  (Figs. 
201  and  202). 

With   this   general   plane   of   arrangement   in   mind   individual 
sections  may  be  studied,  examining  the  arrangement  and  appear- 


244 


THE  PERIDENTAL  MEMBRANE 


ance  of  the  fibers  in  detail.  Figs.  203  and  204  show  the  labial 
and  lingual  gingivre  from  an  incisor  of  a  sheep.  Notice  that  the 
labial  gingiva  is  taller  and  thinner,  and  the  fibers  passing  up  into 
it  are  not  as  strongly  marked.  Notice  also  the  distance  to  which 
the  fine  fibers  of  the  peridental  membrane  can  be  followed  before 
they  are  lost  in  the  coarser  mat  of  gum  tissue.  The  lingual  gin- 
giva is  broader  and  flatter,  and  the  fibers  passing  up  into  it  form  a 
strong  and  well-defined  band.  Under  higher  magnification,  fibers 


1'iG.  204. — Longitudinal  section  of  the  peridental  membrane  in  the  gingival 
portion  (the  lingual  gingivus) :  D,  dentin;  N,  Nasmyth's  membrane;  C,  cementum; 
F,  fibers  supporting  the  gingivus;  Fl,  fibers  attached  to  the  outer  layer  of  the  perios- 
teum over  the  alveolar  process;  f2,  fibers  attached  to  the  bone  at  the  rim  of  the 
alveolus;  D,  bone.  (About  30  X) 

would  be  seen  cut  transversely,  which  pass  around  the  tooth  in  the 
gingiva,  helping  to  hold  it  closely  against  the  enamel.  In  Fig. 
205  the  fibers  uniting  with  the  outer  layers  of  the  periosteum 
are  very  well  shown.  Taking  transverse  sections  in  the  gingival 
portion  and  remembering  that  they  are  cut  at  right  angles  to  these 
through  the  same  area,  the  distribution  of  the  tissues  will  be  better 
understood.  Fig.  206  shows  a  section  cut  close  to  the  gingival 
line.  At  A  the  epithelium  on  the  labial  surface  of  the  gingiva 
is  seen,  and  at  B  the  epithelium  lining  the  gingival  space.  On  the 


ARRANGEMENT 


245 


B     P"«  '     —  ^- 


FIG.  205. — Longitudinal  section  of  peridental  membrane  of  young  sheep,  showing 
fibers  penetrating  the  cementum:  D,  dentin;  C,  cementum,  showing  embedded 
fibers;  F,  fibers  running  to  the  outer  layer  of  the  periosteum,  covering  the  alveolar 
process;  Fl,  fibers  running  to  the  bone  at  the  border  of  the  process;  B,  bone.  (About 
80  X) 


246 


THE  PERIDENTAL  MEMBRANE 


proximal  sides  of  the  roots  the  fibers  will  be  seen  passing  from  the 
cementum  of  one  tooth  to  that  of  the  next.  Fig.  207  is  a  little 
deeper  and  shows  the  fibers  attached  around  the  entire  circum- 
ference of  the  root.  Beginning  at  the  middle  of  the  labial  surface, 
the  fibers  will  be  found  springing  from  the  cementum  and  passing 
out  at  right  angles  to  it,  to  be  lost  in  the  fibrous  mat  supporting 
the  epithelium.  The  fine  fibers  of  the  peridental  membrane  can 
be  followed  for  about  half  the  -distance  to  the  epithelium  before 
they  are  lost  in  the  coarser  mat  of  gum  tissue,  and  a  fairly  definite 
boundary  will  be  seen  between  what  should  be  considered  peri- 


FIG.  206. — Transverse  section  of  the  peridental  membrane  in  the  gingival  portion, 
from  young  sheep.  The  roots  of  two  temporary  incisors  are  cut  across.  The  epithe- 
lium lining  the  gingival  space  is  shown  part  way  around  one.  A,  epithelium  on 
labial  surface  of  gingivse;  B,  epithelium  lining  the  gingival  space.  (About  60  X) 

dental  membrane  and  the  gum  tissue.  As  the  distolabial  angle 
of  the  root  is  approached,  the  fibers  passing  from  the  cementum 
tend  to  swing  around  distally,  and  pass  to  the  mesiolabial  angle 
of  the  adjoining  tooth.  Along  the  proximal  surface  the  network 
which  supports  the  interproximal  gingivus  is  well  shown.  The 
fibers  springing  from  the  cementum  interlace  and  pass  around 
bloodvessels  and  fibers  which  are  passing  up  into  the  gingiva,  and 
finally  are  inserted  into  the  cementum  of  the  next  tooth.  In  this 
way  it  will  be  seen  that  the  teeth  in  the  entire  arch  are  firmly  bound 
together  by  the  fibers  in  the  gingival  portion.  This  explains  the 


PLATE  XVI 

E£5«^**r**T^-~~^C ' 


::  *  w^ 


Transverse  Section  of  Pendental  Membrane. 

Stained  with  hematoxylin  and  eosin.     Alveolar  portion. 


ARRANGEMENT 


247 


way  in  which  the  positions  of  all  the  teeth  are  affected  by  the  loss 
of  a  single  one  in  the  arch,  and  the  way  in  which  the  movement  of 
one  tooth  will  draw  its  neighbors  after  it.  It  also  explains  the 
separation  of  the  central  incisors  when  the  frenum  labium  passes 
through  between  the  teeth,  and  is  inserted  on  the  lingual  surface 
of  the  alveolar  process.  If  these  incisors  are  to  be  held  together 
permanently,  normal  attachment  of  fibers  extending  from  the 


FIG.  207. — Transverse  section  of  the  peridental  membrane  in  the  gingival  portion 
(from  sheep):  E,  epithelium;  F,  fibrous  tissue  of  gum;  B,  point  where  peridental 
membrane  fibers  are  lost  in  fibrous  mat  of  the  gum;  P,  pulp;  F',  fibers  extending 
from  tooth  to  tooth.  (About  30  X) 


cementum  of  one  tooth  to  that  of  the  other  must  be  secured.  The 
fibers  in  this  area  are  also  well  shown  in  Fig.  208,  and  it  can  be 
understood  how  they  form  foundation  upon  which  the  interproximal 
gingiva  rests.  The  first  step  in  the  sagging  of  the  interproximal 
gum  tissue  is  the  cutting  off  of  the  fibers  from  the  cementum,  where 
it  bends  occlusally,  following  the  curve  of  the  gingival  line  on  the 
proximal  surface. 


248 


THE  PERIDENTAL  MEMBRANE 


FIG.  208. — A  portion  of  the  peridental  membrane  between  two  incisors  of  a  young 
sheep,  showing  the  fibers  extending  from  tooth  to  tooth. 


FIG.  209. — Fillers  at  the  border  of  the  alveolar  process  (from  sheep):  D,  dentin; 
C,  cemontum;  F,  filters  extending  from  cementum  to  bone;  Bl,  bloodvessel;  B, 
bone.  (About  80  X) 


PLATE  XVII 


Cm  - 


Al 


-      \ 


Transverse  Section  of  the  Pendental  Membrane  in  the 
Occlusal  Third  of  the  Alveolar  Portion  (from  Sheep). 

V    muscle  fibers;    I'.r,  periosteum;    Al,  bone  of  the  alveolar  process,    Pd.  peri- 
dental  membrane  fibers;    P,  pulp;    D,  clentin;    Cm,  cementum. 


PLATE  XVIII 


Diagram  of  Peridental  Membrane. 

.V,  muscle  fibers;  Per,  periosteum;    D.  clentin;    P.  pulp;    Cm,  cementum;    P<1, 
peridental  membrane  fibers;    Al,  bone  of  the  alveolar  process. 


ARRANGEMENT  249 

i 

Plate  XVII  shows  a  transverse  section  in  the  occlusal  third  of  the 
alveolar  portion  from  the  incisor  of  a  sheep.  Upon  the  labial  a 
few  muscle  fibers  are  seen  and  the  periosteum  covering  the  labial 
surface  of  the  process.  Notice  the  medullary  spaces  in  the  bone 
and  the  canals  opening  into  the  peridental  membrane  and  perios- 
teum. The  light  line  forming  the  outer  boundary  of  the  dentin  is 
characteristic.  Two  layers  of  cementum  are  seen,  and  notice 
the  thickening  of  the  layer  where  strong  bundles  are  attached. 
At  the  middle  of  the  labial  surface  the  fibers  pass  at  right  angles 
to  the  cementum  and  are  attached  to  the  bone,  but  as  the  distolabial 
angle  of  the  root  is  approached  the  bundles  swing  distally  to  be 
attached  in  the  bone.  In  Plate  XVIII,  which  was  drawn  very  care- 
fully from  this  section,  the  arrangement  of  the  fibers  is  shown  dia- 
grammatically.  Notice  the  way  in  which  they  pass  over  and 
under  each  other  and  around  the  bloodvessels  which  wind  through 
them.  This  relation  to  the  bloodvessels  is  important,  and  will  be 
considered  again  later  in  connection  with  the  blood  supply  of  the 
membrane.  The  tangential  fibers  at  the  angle  of  the  root  hold 
the  tooth  against  the  forces  which  tend  to  rotate  it  in  its  socket. 
They  are  important  in  connection  with  all  rotating  movements  in 
orthodontia.  It  has  long  been  noted  that  rotations  were  the  hardest 
movements  to  retain,  especially  if  the  tooth  were  moved  in  no 
other  direction.  In  this  case,  if  the  tooth  were  turned  mesially 
the  fibers  at  the  distolabial  angle  would  spring  the  thin  plate  of  the 
alveolar  process  as  a  bow  is  bent,  leaving  a  condition  of  stress  in 
the  tissue  which  will  tend  to  spring  back  into  its  old  position  and 
drag  the  tooth  with  it.  Notice  the  greater  thickness  of  the  mem- 
brane on  the  lingual  as  compared  with  the  labial.  Figs.  205  and 
209  show  longitudinal  sections  at  the  border  of  the  alveolar  process. 
Notice  that  the  fibers  can  be  seen  running  through  the  entire  thick- 
ness of  the  cementum.  They  are  large,  strong  fibers  and  branch 
very  little  in  their  course.  Note  the  bloodvessel  that  is  shown  in 
several  of  these  sections,  and  the  way  in  which  it  gives  off  branches 
passing  over  the  border  of  the  processes  and  toward  the  cementum. 


CHAPTER  XXI. 

THE  CELLULAR  ELEMENTS  OF  THE  PERIDENTAL 
MEMBRANE. 

Fibroblasts. — The  fibroblasts  are  found  everywhere  between  the 
fibers  which  they  have  formed  and  to  which  they  belong.  They 
are  spindle-shaped  or  stellate  connective-tissue  cells,  having  a 
more  or  less  flattened  nucleus  and  a  body  of  granular  cystoplasm, 


FIG.  210.— Fibers  and  fibroblasts  from  transverse  section  of  membrane:     F,  fibers 
cut  transversely;     F1,  fibers  cut  longitudinally,  showing  fibroblasts.      (About  80  X) 

which  is  squeezed  out  into  thin  projections  between  the  fibers. 
In  sections  stained  with  hematoxylin  the  cells  take  the  stain  strongly 
and  the  fibers  remain  clear  (Fig.  210).  In  this  way  the  fibers  are 
marked  out  by  the  cells  which  lie  between  them.  The  number  of 
the  fibroblasts  in  the  membrane  decreases  with  age.  They  are 
large  and  numerous  in  the  membrane  of  a  newly  erupted  tooth  and 
are  comparatively  small  and  few  in  the  membrane  around  an  old 
tooth.  This  is,  however,  characteristic  of  fibroblasts  in  connective 
tissue  generally.  Fig.  210  shows  a  small  field  taken  from  the  gingival 
portion  of  the  membrane  between  the  teeth.  The  magnification 
is  low,  the  photograph  being  made  with  a  f  objective.  The  cells 
(250) 


251 


are  seen  as  little  dark  dots  lying  between  the  fibers,  which  are 
clear.  Where  the  fibers  are  cut  longitudinally  they  appear  spindle- 
shaped,  but  where  the  fibers  are  cut  across  they  appear  star-shaped. 
They  will  be  seen  better  in  photographs  made  with  higher  magnifi- 
cation, but  an  adequate  idea  of  their  form  can  only  be  obtained 
by  studying  sections  very  carefully  with  a  |  or  y1^  objective  and 
using  the  fine  adjustment  to  gain  an  idea  of  the  third  dimension 
of  space.  They  are  shown  in  many  of  the  illustrations  of  the 
epithelial  structures. 

Cementoblasts. — The  cementoblasts  are  the  cells  which  form 
cementum.  They  cover  the  surface  of  the  root  everywhere  between 
the  fibers  which  are  embedded  in  the  tissue.  While  these  cells 
perform  the  same  function  for  the  cementum  as  the  osteoblasts 
do  for  bone,  they  are  quite  different  in  form.  They  are  always 


FIG.  211. — -Isolated  cementoblasts, 
showing  the  form  of  the  cell  as  it  fits 
around  the  fibers  springing  from  the 
cementum.  (Black.) 


FIG.  212. — Cementoblasts  as  seen  in 
a  section  at  a  tangent  to  the  root  and 
just  missing  the  cementum.  The  fibers 
are  left  white,  the  cells  are  shaded. 
(Black.) 


flattened  cells,  sometimes  almost  scale-like,  and  when  seen  from 
above,  very  irregular  in  outline.  This  irregularity  in  outline  is 
due  to  the  projections  of  the  cytoplasm  around  the  fibers  as  they 
spring  from  the  cementum,  the  edges  of  the  cell  being  notched  and 
scalloped  to  fit  about  them.  There  is  a  central  mass  of  granular 
cytoplasm  which  contains  an  oval  and  more  or  less  flattened 
nucleus,  from  which  the  cytoplasm  extends  in  projections  passing 
partly  around  the  fibers.  Isolated  cementoblasts  are  shown  in 
Fig.  211,  drawn  by  Dr.  Black.  In  order  to  obtain  an  idea  of  the 
form  of  the  cementoblasts,  sections  must  be  cut  at  a  tangent  to 
the  surface  of  the  root,  and  just  missing  the  surface  of  the  cemen- 
tum. In  this  way  the  fibers  are  cut  across  andi  the  cementoblasts 


252  THE  PERI  DENTAL  MEMBRANE 

are  shown  covering  the  entire  surface  between  the  fibers.  These 
are  shown  in  Fig.  212,  in  which  the  fibers  are  left  perfectly  clear 
in  order  to  outline  the  cells  more  distinctly.  In  sections  cut  at 
right  angles  to  the  surface  of  the  roots  (Figs.  223,  224,  and  225) 
the  cementoblasts  are  shown  as  more  or  less  flattened,  but  no 
idea  of  the  way  in  which  they  fit  about  the  fibers  can  be 
obtained. 

Cytoplasmic  processes  extend  from  the  body  of  the  cemento- 
blasts into  the  matrix  of  the  cementum.  These  correspond  to  the 
process  of  the  osteoblasts  which  occupy  the  canaliculi  of  bone. 
They,  however,  are  not  nearly  as  numerous  or  as  regular  in  their 
arrangement  as  the  osteoblasts.  Processes  extending  from  these 
cells  in  a  direction  from  the  cementum  out  into  the  tissue  of  the 
membrane  have  not  been  demonstrated. 

Cement  Corpuscles. — Occasionally  a  cementoblast  becomes  fast- 
ened down  to  the  surface  and  enclosed  in  the  matrix  that  is  formed. 
They  then  lie  in  a  lacuna  and  show  processes  radiating  from  them 
into  the  canaliculi.  These  correspond  to  bone  corpuscles,  but 
there  is  no  such  regularity  of  their  disposition  or  arrangement  with 
reference  to  the  lamellae,  as  is  shown  in  the  case  of  bone.  In  man 
the  cementum  in  the  gingival  half  of  the  root  is  usually  without 
cement  corpuscles.  They  often  lie  entirely  within  a  single  lamella 
instead  of  between  two,  as  is  the  case  in  bone.  In  general  they  are 
found  where  the  layers  are  thick  and  the  embedded  fibers  are 
not  specially  numerous.  They  are  very  often  seen  where  absorp- 
tions have  been  refilled  by  the  formation  of  subsequent  layers 
(Figs.  140  and  141). 

It  is  by  the  activity  of  the  cementoblasts  producing  a  new  layer 
of  cementum  that  the  fibers  are  attached  to  the  surface  of  the 
root.  In  studying  many  sections,  places  are  found  wrhere  the 
fibers,  though  lying  in  contact  with  the  surface  are  not  attached 
to  the  cementum.  In  some  places  it  can  be  seen  that  they  have 
been  cut  off  by  absorptions.  From  a  study  of  these  layers  it  is 
evident  that  there  is  a  constant  readjustment  in  the  attachment  of 
the  fibers  to  the  root  during  the  function  of  the  tooth,  which  prob- 
ably adapt  it  to  slight  changes  of  position  resulting  from  wear  and 
other  conditions.  It  is  important  to  remember  that  whenever 
the  fibers  have  been  stripped  from  the  surface  of  the  cementum, 
they  can  be  reattached  to  it  only  by  the  formation  of  a  new  layer 
of  cementum,  building  the  fibers  into  it.  This  is  certainly  possible 
if  the  conditions  are  properly  controlled,  but  the  cells  of  the  tissue 


CEMENT  CORPUSCLES 


253 


must  be  in  a  normal  and  vitally  active  condition,  and  the  surface 
of  the  root  must  be  such  that  they  can  lie  in  physiological  contact 


Pd.M 


Ms 


Pd.B 


H.B 


P'iG.  213. — Penetrating  fibers  in  bone.  A  field  from  Plate  XIX:  Pd.M,  pcridental 
membrane;  Ob1,  osteoblasts  of  peridental  membrane;  Ol2,  osteoblasts  of  medullary 
space;  Pd.B,  solid  subperidental  and  subperiosteal  bone  with  embedded  fibers; 
Ms,  medullary  space  formed  by  absorption  of  the  solid  subperidental  bone  with 
embedded  fibers;  H.B,  Haversian  system  bone  without  fibers  built  around  the 
medullary  space.  (About  200  X) 

with  it.  The  cure  of  a  pyorrhea  case  therefore  becomes  a  biological 
problem.  In  this  connection  it  is  important  to  remember  that  a 
surface  of  cementum  which  has  long  been  bathed  in  pus  may  be  so 


254  THE  PERIDENTAL  MEMBRANE 

filled  with  poison  that  no  cell  can  lie  in  contact  with  it  and  perform 
its  functions. 

Osteoblasts. — The  osteoblasts  of  the  peridental  membrane  are 
exactly  like  osteoblasts  in  other  positions.  They  cover  the  surface 
of  the  bone  of  the  alveolar  wall  lying  between  the  fibers  which 
are  embedded  in  it.  Even  in  the  young  subject  they  are  not  found 
in  every  position,  while  in  an  adjoining  area  the  surface  of  the  bone 
may  be  covered  with  them.  In  the  old  subject  they  are  generally 
absent  or  have  been  reduced  to  flattened  scales,  which  are  very 
difficult  to  demonstrate;  but  even  in  these  cases  areas  will  be 
found  in  which  osteoblasts  are  present.  These  are  areas  of  active 
bone  formation.  The  osteoblasts  lay  down  bone  exactly  as  occurs 
in  attached  portions  of  the  periosteum,  but  after  a  little  thickness 
of  this  solid  peridental  bone  has  been  formed  it  is  perforated  by 
penetrating  canals,  on  the  walls  of  which  absorptions  occur,  form- 
ing spaces  about  which  new  Haversian  system  bone  is  formed. 
This  is  illustrated  in  Plate  XIX.  In  this  way  only  sufficient  sub- 
peridental  bone  is  left  to  furnish  an  attachment  for  the  fibers. 

Fig.  213  shows  a  higher  magnification  of  a  small  area.  The 
osteoblasts  are  seen  between  the  fibers  on  the  surface  of  the  alveolus, 
and  the  fibers  can  be  followed  through  the  subperidental  bone. 
A  large  absorption  area  has  been  formed  which  has  been  partly 
rebuilt,  and  the  new-formed  bone  without  embedded  fibers  is 
lighter  in  color.  An  understanding  of  this  building  and  rebuilding 
of  bone  through  the  agency  of  the  peridental  membrane  is  neces- 
sary to  understand  the  development  of  the  face  and  everything  in 
connection  with  tooth  movement,  whether  physiological  or  artificial. 

Osteoclasts. — The  osteoclasts  of  the  peridental  membrane  are 
not  constant  elements.  They  appear  and  disappear  in  response 
to  the  same  conditions  which  lead  to  their  appearance  and  disap- 
pearance in  bone.  They  are  always  large,  multinuclear  cells,  having 
from  three  or  four  to  thirty  of  forty  nuclei  (Fig.  214).  They  may 
appear  upon  the  surface  of  the  cementum,  upon  the  surface  of  the 
alveolar  wall,  or  within  the  medullary  spaces  of  the  bone.  They 
are  formed  from  embryonal  cells  in  the  tissue  in  response  to  mechan- 
ical stimuli.  Morphologically  they  are  in  no  respect  different  from 
the  osteoclasts  in  bone. 

The  osteoclasts  are  tissue  destroyers  and  are  the  active  agents 
in  the  removal  of  any  hard  tissue.  There  is  no  difference  in  them, 
whether  they  are  destroying  the  fibrous  tissue,  bone,  cementum, 
or  dentin  (Fig.  215).  In  order  for  them  to  act,  their  cytoplasm 


PLATE  XIX 


PdB 


Border  of  Growing  Process. 

Cm,  cementum;  P<1,  peridental  membrane;  PdB,  solid  subpericlental  and 
subperiosteal  bone  with  embedded  fibers;  .1/8,  medullary  space  formed  by 
absorption  of  the  solid  bone;  //  B,  Haversian  system  bone  without  fibers; 
Per,  periosteum.  (About  SO  X-) 


OSTEOCLASTS 


255 


must  lie  in  actual  contact  with  the  surface  to  be  attacked.  They 
do  not  first  decalcify  and  then  remove,  but  apparently  by  applying 
their  cytoplasm  to  its  surface  the  cells  destroy  the  intercellular 
substance,  forming  hollows  in  the  surface,  into  which  the  cells 
sink.  These  hollows  have  been  called  Howship's  lacunae.  The 
cells  usually  appear  in  groups  and  spread  out  over  the  bone  or 
cementum  to  be  attacked,  but  sometimes  only  two  or  three  will 


FIG.  214. — Osteoclast  absorption  of  bone  over  permanent  tooth:    Oc,  osteoclasts;    B , 
bone  of  crypt  wall;  F,  fibrous  tissue  of  follicle  wall;  A,  ameloblasts.    (About  62  X) 

be  found  at  a  point  on  the  surface  of  the  bone,  and  these  will  burrow 
into  the  substance,  forming  a  penetrating  canal  running  through 
the  bone  (Figs.  216  and  217).  In  these  positions  the  osteoclasts 
are  usually  comparatively  small.  As  fast  as  the  canal  is  formed 
the  embryonal  cells  of  the  membrane  multiply  and  grow  into  the 
space  and  at  any  point  where  absorption  is  going  on  the  portion 
destroyed  is  immediately  replaced  by  embryonal  connective  tissue. 
This  will  be  noted  in  all  the  illustrations  showing  absorptions. 


256 


Whenever  absorption  is  going  on  formation  is  also  going  on  in  an 
adjoining  area.  In  this  way  the  function  of  the  tissue  is  maintained 
until  the  last  remnants  of  it  are  destroyed.  The  general  statement 
may  be  made  that  bone  formation  is  always  accompanied  by  bone 
destruction,  and  bone  destruction  by  rebuilding.  The  result  depends 


FIG.  215. — Ostcoclasts  in  cancellous  bone  near  the  peridental  membrane;  in  some 
portions  of  the  field  osteoblasts  are  seen.  As  bone  is  removed  note  how  embryonal 
connective  tissue  replaces  it. 


upon  which  side  the  balance  swings.  The  alternation  of  formation 
and  absorption  in  the  removal  of  hard  tissues  is  well  illustrated 
in  the  absorption  of  the  roots  of  the  temporary  teeth.  The  absorp- 
tion does  not  begin  at  one  point  and  spread  continuously  over  the 
entire  surface  of  the  root.  If  it  did  so,  all  of  the  fibers  would  be 


OSTEOCLASTS 


257 


&m  •/•.*•:•;• 

'    ^IlKfc. 


FIG.  216. — Osteoclast  absorption  forming  penetrators  of  canal:  a,  bone  matrix; 
6,  bloodvessel;  c,  embryonal  connective  tissue;  d,  new  bone  formation;  e,  osteo- 
biasts;  /,  osteoclasts.  (Black.) 


FIG.   217. — A  longitudinal  section  through  the  remains  of  the  alveolar  process 
around  the  root  of  a  temporary  tooth  about  to  be  shed  (sheep) :  C,  the  cementum  on 
the  remains  of  the  tooth;    B,  penetrating  canals  cut  through  the  labial  plate  of  bone. 
17 


258 


THE  PERI  DENTAL  MEMBRANE 


cut  out  and  the  tooth  would  drop  off  with  at  least  a  considerable 
portion  of  the  root.     The  process  progresses  in  something  of  this 


Cm1 


FIG.  218. — Root  of  a  temporary  incisor,  showing  absorption  and  rebuilding  of 
rementum  (from  sheep):  G,  gingivus;  D,  den  tin;  Cm,  cementum;  Ab,  absorption 
cavity,  showing  Howship's  lacunae;  Cm1,  new-formed  cementum.  (About  50  X) 

fashion:    At  a  point  on  the  side  of  the  root  near  the  apex,  where 
the  growth  of  the  erupting  tooth  produces  pressure,  osteoclasts 


OSTEOCLASTS  259 

appear  in  the  membrane,  cutting  off  the  fibers,  displacing  the 
cementoblasts,  and  arranging  themselves  in  groups  on  the  surface 
of  the  root.  These  dissolve  away  the  cementum  and  sink  into 
the  tissue,  perhaps  cutting  into  the  dentin  for  a  short  distance. 
By  this  excavation  the  pressure  is  relieved,  the  osteoclasts  disap- 
pear, cementoblasts  are  formed  in  the  embryonal  connective  tissue, 


FIG.  219. — A  transverse  section  through  an  incisor  from  the  same  jaw  as  Fig.  218, 
and  at  the  level  of  Cm1,  showing  the  refilling  of  the  absorption  cavity  by  new  layers 
of  cementum. 

and  the  deposit  of  cementum  begins  in  the  excavation,  reattaching 
the  fibers  in  this  area.  As  the  rebuilding  progresses,  at  a  point  a 
little  farther  occlusally,  osteoclasts  appear  and  begin  a  new  excava- 
tion. In  this  manner  the  process  continues.  ^Yhen  the  absorption 
stops  in  the  second  point,  it  begins  again  at  the  first,  cutting  much 
deeper  into  the  dentin,  and,  oscillating  back  and  forth,  it  progresses 
until  all  of  the  dentin  may  be  destroyed,  leaving  the  hollow  cap  of 


260  THE  PERIDENTAL  MEMBRANE 

enamel,  and  even  then  new-forming  cementum  to  maintain  the 
attachment  will  be  found  around  the  circumference.  In  this  way 
it  will  be  seen  that  the  function  of  the  tooth  is  maintained  until 
its  successor  is  ready  to  take  its  place  in  a  very  short  time.  The 
importance  of  this  arrangement  will  be  more  fully  appreciated 
after  a  study  of  the  relation  of  the  teeth  to  the  development  of 
the  face.  Fig.  218  shows  a  longitudinal  section  through  a  temporary 
incisor  of  a  sheep.  At  Ab  an  absorption  has  just  been  completed, 
for  the  osteoclasts  have  disappeared.  The  excavation  is  seen 
filled  with  embryonal  tissue  and  rebuilding  is  about  to  begin.  At 
Cm  an  older  and  much  larger  absorption  space  is  seen  which  has 
been  partially  replaced  by  a  formation  of  new  cementum  reattach- 
ing  fibers.  In  Fig.  219  a  transverse  section  of  the  root  is  seen  which 
is  from  the  same  jaw  cut  at  the  level  of  Cm,  and  shows  the  absorp- 
tion refilled.  This  patchwork  performance  goes  on  in  the  same 
way  in  the  bone  of  the  alveolar  process,  and  its  study  is  one  of  the 
most  interesting  phases  of  the  relation  of  the  teeth  to  the  develop- 
ment of  the  face.  Without  a  clear  idea  of  this  it  is  impossible  to 
understand  how  the  teeth,  after  their  roots  are  fully  formed,  can 
move  through  three  dimensions  of  space  and  retain  their  function 
all  the  time. 

Epithelial  Structures. — The  epithelial  structures  of  the  peridental 
membrane  were  first  described  by  Dr.  Black  in  his  volume  Perios- 
teum and  Peridental  Membrane,  published  in  1887.  At  this  time 
Dr.  Black  considered  them  to  be  of  lymphatic  character  and  named 
them  endolymphatics.  His  conception  of  them  was  that  they  were 
lymphatic  channels  crowded  with  adenoid  cells.  Since  then  the 
form  and  appearance  of  the  cells  and  the  character  of  their  reaction 
with  staining  agents  has  shown  the  cells  to  be  of  epithelial  character. 
In  the  same  year  that  Dr.  Black's  book  was  published,  von  Brunn1 
described  the  same  structures.  He  considered  them  as  epithelial 
remains  of  the  outer  layer  of  the  enamel  organ,  growing  down 
around  the  root  beyond  the  gingival  line  where  the  formation  of 
enamel  stops.  It  has  seemed  probable  to  the  writer  that  this 
was  correct,  but  their  histogenesis  has  not  been  sufficiently  well 
followed,  and  it  presents  an  attractive  field  for  research.  These 
structures  undoubtedly  originate  from  the  epithelium  of  the  enamel 
organ,  probably  both  of  the  outer  and  inner  layers.  In  the  authors' 
opinion  they  have  an  important  relation  to  the  formation  of  ceinen- 

1  Archiv  f.  Anatomic,  1887. 


DISTRIBUTION 


261 


turn  which  accounts  for  their  persistence  in  the  membrane.  While 
they  are  derived  from  an  embryonal  structure,  the  enamel  organ, 
it  does  not  seem  proper  to  regard  them  as  embryonal  remains,  for 
while,  like  all  the  cellular  elements,  they  are  more  numerous  in 
young  people  than  in  old,  they  are  persistent  throughout  life.  They 
have  been  shown  in  the  membrane  from  a  man  aged  seventy  years, 
and  it  does  not  seem  logical  to  suppose  that  embryonal  debris  that 
was  useless  to  the  organism  would  persist  through  life.  Up  to  the 
present  time,  however,  nothing  has  been  discovered  about  these 
structures  to  throw  any  light  upon  their  function.  The  author  has 
long  been  of  the  opinion  that  they  were  related  to  the  formation  of 
cementum  but  this  is  not  sufficiently  established  to  be  more  than 
suggested  here.  Specimens  have  strongly  indicated  that  they  were 
important  in  some  pathologic  conditions.  Their  cells  have  been 
found  dead  and  degenerating  in  pathologic  material  beyond  the 
point  showing  any  pathologic  condition  in  other  cells.  These  struc- 
tures have  been  observed  in  sections  from  man,  sheep,  cat,  dog,  and 
monkey.  The  best  material  for  their  study  is  a  young  sheep  or  pig. 


FIG.  220. — Diagram  of  glands  of  peridental  membrane.     (Black.) 

Distribution. — These  structures  are  composed  of  cords  or  rows 
of  epithelial  cells,  surrounded  by  an  extremely  delicate  basement 
membrane  (Fig.  220).  In  some  cases  there  is  a  slight  indication 
of  a  circular  arrangement  of  connective  tissue  around  them.  The 
cords  lie  very  close  to  the  surface  of  the  cementum,  winding  in 
and  out  among  the  fibers  (Fig.  221).  They  anastomose  and  join 


262 


THE  PERIDENTAL  MEMBRANE 


with  each  other,  forming  a  network  the  meshes  of  which  are  com- 
paratively close  in  the  gingival  portion  (Fig.  222),  and  compara- 
tively wide  in  the  apical  portion,  the  cords  becoming  scarcer  as  the 


FIG.  221. — A  section  cutting  diagonally  through  the  root,  showing  the  network  of 
epithelial  cords,  A ;   dentin,  D ;   cementum,  Cm 


apex  of  the  root  is  approached,  but  the  author  has  seen  them  in 
sections  from  the  apical  third. 

A  binocular  microscope  was  used  to  obtain  a  true  conception 


THE  ARRANGEMENT  OF  THE  CELLS 


263 


of  the  way  in  which  these  cords  wind  in  and  out  among  the  bundles 
of  fibers.  The  cords  show  a  marked  tendency  to  run  out  into  the 
membrane  and  loop  back  (Fig.  223),  coming  very  close  to  the 
surface  of  the  cementum. 

The  ends  of  the  loops  toward  the  cementum  often  show  enlarge- 
ments which  in  some  cases  apparently  lie  directly  in  contact  with 
the  cementum  (Figs.  224  and  225).  These  enlargements  next  to 
the  cementum  are  shown  in  Fig.  223. 


FIG.  222. — Transverse  section  of  the  peridental  membrane  in  the  gingival  portion, 
showing  the  position  of  the  epithelial  cords.  At  1  the  loop  shown  in  higher  mag- 
nification in  Fig.  224  is  seen. 

The  Arrangement  of  the  Cells. — There  is  no  definite  arrangement 
of  the  cells  in  these  cords.  In  some  places  there  will  be  a  ring  of 
irregular  polyhedral  or  rounded  cells  which  almost  exactly  resemble 
a  simple  tubular  gland.  In  other  places  there  is  a  pretty  definite 
outer  ring  of  cells  and  a  central  mass  enclosed  by  them.  The  cells 
are  made  up  of  granular  cytoplasm,  each  containing  an  ovoid 
nucleus  that  is  rich  in  chromatin.  The  author  has  spent  much 
time  attempting  to  work  out  the  relation  of  these  cords  to  the 
epithelium  lining  the  gingival  space,  thinking  that  possibly  they 
open  into  it.  In  a  few  places  structures  appearing  very  much  like 


264 


THE  PERIDENTAL  MEMBRANE 


a  duct  have  been  seen,  as  shown  in  Fig.  227,  but  they  are  appar- 
ently only  unusually  large  cords.  There  is  no  regularity  in  places 
where  they  are  found,  and  no  connection  with  the  gingival  space 
has  ever  been  discovered.  Toward  the  gingival,  as  the  gingival 
line  is  approached,  the  cords  seem  to  swing  out  away  from  the 
cementum,  especially  on  the  proximal  side,  and  to  pass  up  into  the 
gingivse,  where  they  are  lost  among  the  projections  of  the  epithe- 
lium. 


EC 


FIG.  223. — Epithelial  structures  of  the  peridental  membrane  (from  sheep) :  Fb, 
fihroblasts;  .Ec,  epithelial  structures;  Cb,  cementoblasts;  Cm,  cementum;  D,  dentin. 
(About  468  X) 

Gland  of  Serres. — Salter,  in  his  Dental  Pathology  and  Surgery, 
quotes  Serres,  who  assigns  the  function  of  a  gland  to  the  epithelium 
lining  the  gingival  space.  This,  the  writer  believes,  is  the  first 
reference  to  an  appearance  in  the  tissues  that  has  been  called  the 
gland  of  Serres.  It  has  long  been  noted  that  the  epithelium  lining 


GLAND  OF  SERRES 


265 


the  gingival  space  was  lighter  in  structure,  composed  of  larger 
cells,  and  had  no  horny  layer  on  its  surface,  as  is  true  of  the  epithe- 


FIG.  224. — Epithelial  structures:    EC,  epithelial  cord,  apparently  showing  a  lumen; 
Cb,  cementoblasts;    Cm,  cementum;    D,  dentin.     This  loop  is  seen  in  Fig.  209. 


FIG.  225. — Transverse  section,  showing  the  cellular  elements.     (About  900  X) 


266 


THE  PERIDENTAL  MEMBRANE 


Hum  on  the  outer  surface  of  the  gingivus.  Upon  the  proximal 
surfaces  the  projections  of  the  epithelium  which  extend  down 
between  the  papillae  of  connective  tissue,  which  constitute  the 
stratum  papillaris,  are  specially  long,  and  in  the  connective  tissue 
between  them  collections  of  small  round  cells  are  often  found.  It 
is  between  these  projections  of  epithelium  that  the  cords  of  epithe- 


FIG.  226. — Epithelial  structures  (from  sheep):  Fb,  fibroblasts;   EC,  epithelial  struct- 
ures;   Cb,  cemen  to  blasts;    Cm,  cementum;   D,  dentin.     (About  700  X) 

Hal  cells  which  have  been  described  are  lost,  and  to  this  portion 
of  the  tissue  Dr.  Black  has  again  called  attention,  as  the  gland 
of  Series.  Sufficient  work  has  not  yet  been  done  upon  this  subject 
to  know  whether  this  is  a  constant  arrangement,  or  whether  it  is 
found  only  in  certain  animals,  or  even  whether  it  may  not  possibly 
be  pathologic.  The  appearance  is  shown  in  Figs.  228  and  229. 
The  work  of  the  last  five  years  has  convinced  the  writer,  that 
this  appearance  is  the  reaction  of  the  tissue  to  infection.  One 


BLOODVESSELS  267 

of  the  important  functions  of  the  supporting  tissues  about  the 
necks  of  the  teeth  is  to  resist  and  remove  infection,  and  all  the 
structural  elements  of  the  tissue  are  arranged  for  that  function. 
The  epithelium,  the  connective-tissue  fibers,  the  capillary  blood- 
vessels, the  lymphatics,  and  the  cellular  elements  of  the  connective 
tissue,  are  so  arranged  as  to  immediately  respond  to  an  invasion  of 
infecting  organisms  in  such  a  way  as  to  destroy  and  remove  them  if 
possible. 


FIG.  227. — A  very  large  cord  which  was  at  first  mistaken  for  a  duct. 

Bloodvessels. — The  peridental  membrane  possesses  a  very  rich 
blood  supply.  A  number  of  vessels  enter  the  membrane  in  the 
apical  portion  from  the  medullary  spaces  in  the  bone.  Some  of 
these,  passing  through  canals  in  the  apex  of  the  root,  supply  the 
dental  pulp,  others  pass  up  through  the  membrane.  As  they 
extend  occlusally  they  give  off  and  receive  branches  which  enter 
the  membrane  from  the  bone  of  the  alveolar  wrall.  In  this  way 
the  caliber  of  the  principal  vessels  is  maintained  throughout  their 
course  in  the  membrane.  As  they  reach  the  border  of  the  alveolar 
process  they  give  off  branches  which  anastomose  with  the  vessels 
of  the  periosteum  and  gum  tissue. 

In  the  young  membrane  these  vessels  occupy  a  position  closer  to 
the  bone  than  the  cementum,  and  as  the  membrane  becomes  thinner 
they  often  come  to  lie  in  grooves  in  the  bone.  Vessels  of  any  size 


268 


THE  PERJDENTAL  MEMBRANE 


are  rarely  seen  close  to  the  cementum,  and  the  capillaries  in  the 
membrane  are  rather  scarce,  though  they  are  more  numerous  than 


FIG.  228. — Longitudinal  section,  cut  mesiodistally:  D,  dentin;  Cm,  cementum 
which  has  separated  from  the  dentin;  Gs,  gingival  space;  Ep,  epithelial  projection 
from  the  lining  of  the  gingival  space;  EC,  epithelial  cords;  Rs,  small  round  cells  in 
the  connective  tissue. 

in  most  connective  tissues  of  as  compact  a  character.  The  anas- 
tomosis of  the  vessels  in  the  membrane  is  quite  rich.  It  is  impor- 
tant to  remember  that  the  cancellous  bone  of  the  process  is  richly 


BLOODVESSELS 


269 


supplied  with  bloodvessels,  and  the  anastomosis  with  the  vessels 
of  the  membrane,  from  the  alveolar  wall  and  over  the  border  of 
the  process,  is  important  in  the  consideration  of  pathologic  condi- 


FIG.  229. — A  longitudinal  section  cut  mesiodistally:  E,  epithelium  of  the  gingivus; 
Gs,  gingival  space;  Cm,  cementum  which  has  separated  from  the  den  tin;  EC,  epithe- 
lial cords. 

tions.  Iii  alveolar  abscess  the  vessels  entering  through  the  apical 
space  may  be  entirely  cut  off,  but  this  does  not  disturb  the  blood 
supply  of  the  rest  of  the  membrane.  The  removal  of  the  pulp  has 


270  THE  PERIDENTAL  MEMBRANE 

often  been  advocated  in  the  treatment  of  pathologic  conditions 
of  the  membrane,  on  the  ground  that  the  vessels  entering  the  pulp 
rob  the  membrane  of  blood  supply,  and  that  their  removal  made 
recovery  more  certain.  No  one  having  a  knowledge  of  the  blood 
supply  of  the  membrane  could  advise  this  for  that  reason. 

In  their  course  through  the  membrane  the  vessels  wind  between 
the  principal  fibers  in  a  way  that  can  only  be  appreciated  by  study- 
ing sections  with  a  binocular  instrument,  and  when  this  condition 
is  realized  it  can  be  understood  how  some  inflammations  in  the 
membrane  are  set  up.  For  instance,  when  force  is  applied  to  a 
tooth,  the  principal  fibers  are  stretched.  This  causes  them  to  close 
some  spaces  and  open  others.  The  vessels  in  the  closed  spaces  are 
constricted  and  the  flow  of  blood  through  them  partly  shut  off. 
The  vessels  in  the  enlarged  spaces  dilate  to  compensate.  If  the 
force  is  removed,  the  dilated  vessels  are  again  constricted,  and  the 
constricted  ones  enlarged,  and  the  result  is  a  literal  sawing  upon 
the  walls  of  the  bloodvessels  which  in  a  very  short  time  will  set  up 
an  acute  inflammation.  This  is  extremely  important  in  the  applica- 
tion of  force  in  orthodontia,  and  often  also  in  the  use  of  the  mallet 
in  condensing  gold,  especially  for  young  patients. 

Lymphatic  Vessels. — The  lymphatics  of  the  peridental  membrane 
have  been  described  in  Chapter  XIV.  The  writer  was  first  convinced 
of  their  presence  in  the  membrane  by  a  study  of  the  manner  of 
extension  of  destructive  inflammations  of  the  peridental  membrane 
and  they  were  afterward  demonstrated  by  injection.  The  collecting 
vessels  from  the  labial,  buccal  and  lingual  surfaces  of  the  gums  and 
gingivse  pass  outside  of  the  periosteum  of  the  alveolar  process  to 
the  wreath  of  collecting  trunks  at  the  reflection  of  the  tissues  from 
the  surface  of  the  bone  to  the  lips  or  cheeks  on  the  outside  and  to 
the  collecting  trunks  of  the  floor  of  the  mouth  and  palate  on  the 
inside.  The  collecting  vessels  from  the  papillae  lining  the  gingival 
spaces  penetrate  the  ligamentum  circulare  very  close  to  the  cementum 
and  extend  in  the  interfibrous  tissue  accompanying  the  bloodvessels 
and  nerves,  through  the  peridental  membrane  as  far  as  the  apex  of 
the  root,  where  they  anastomose  with  the  efferent  vessels  from  the 
dental  pulp.  They  have  been  followed  through  the  bone  to  the  infra- 
orbital  canal  and  the  inferior  dental  canal  emerging  from  the  corre- 
sponding foramina  and  passing  to  the  lymph  nodes  of  the  submaxil- 
lary  group.  Injected  vessels  in  the  peridental  membrane  are  shown 
in  Fig.  230. 


NERVES 


271 


Nerves. — The  nerves  of  the  peridental  membrane  enter  the 
peridental  membrane  in  company  with  the  bloodvessels.  Their 
source  is  the  same  as  that  of  the  bloodvessels.  The  trunks  entering 
in  the  apical  space  contain  from  eight  or  ten  to  fifteen  or  twenty 
medullated  fibers.  Some  of  these  enter  the  dental  pulp,  others 
extend  up  through  the  membrane,  winding  in  and  out  among  the 
fibers,  generally  following  the  course  of  the  bloodvessels.  Many 
trunks  containing  eight  or  ten  fibers  enter  through  the  alveolar 


FIG.  230. — Transverse  section  of  the  peridental  membrane;  showing  injected 
lymphatic  vessels  (oc.,  3;  obj.,  16  mm.;  reduced  about  one-tenth). 

wall.  In  this  way  a  fairly  rich  plexus  is  formed,  from  which  fibers 
are  continually  given  off  to  be  lost  in  the  tissue.  They  probably 
terminate  in  beaded  free  endings.  No  special  nerve  endings  have 
been  demonstrated.  A  few  Pacinian  corpuscles  have  been  seen 
near  the  gingival  border.  These  are  not  generally  found,  however. 
The  nerves  of  the  membrane  give  to  it  the  sense  of  touch,  which 
is  the  only  sensory  function  of  the  membrane.  As  has  been  noted 
in  connection  with  the  dental  pulp,  the  hard  tissues  and  the  pulp 
have  no  sense  of  touch.  The  contact  of  any  substance  with  the 
surface  of  the  tooth  is  reported  to  consciousness  through  the  medium 


272 


THE  PERIDENTAL  MEMBRANE 


of  the  peridental  membrane.  For  instance,  the  slightest  touch  of 
a  delicate  instrument  produces  a  slight  movement  of  the  tooth 
which  affects  the  nerves  between  the  fibers.  The  delicacy  of  this 
mechanism  can  be  demonstrated  by  the  following  experiment. 
Lightly  touch  the  surface  of  the  enamel  and  the  patient  will  tell 
at  once  not  only  which  tooth  is  touched,  but  whether  a  steel  instru- 
ment or  a  wooden  point  or  some  soft  material  was  used.  If,  how- 
ever, the  finger  is  placed  upon  a  surface  of  the  tooth  and  firm  pressure 
made  in  one  direction,  the  contact  of  the  point  will  not  be  recognized. 


FIG.  231. — -Young  membranes  (from  sheep):  D,  dentin;  Cm,  cementum;  Cm1 
thickening  of  cementum  to  attach  fibers  at  the  corner;  Pd,  peridental  membrane; 
B,  bone  forming  the  wall  of  the  alveolus.  (About  80  X) 

The  Changes  in  the  Peridental  Membrane  with  Age. — The  teeth 
are  formed  in  crypts  in  the  bone,  and  when  they  begin  to  erupt 
the  roof  of  the  crypt  is  removed  by  absorption,  making  an  opening 
large  enough  for  the  crown  to  pass.  As  the  root  is  formed,  the 
tooth  moves  occlusally  and  the  alveolus  grows  up  around  it,  begin- 
ning at  the  margins  of  the  crypt.  When  the  tooth  first  erupts,  there- 
fore, the  alveolus  is  much  larger  than  the  root,  and  the  fibers  of 
the  peridental  membrane  are  very  long.  The  size  of  the  alveolus 
is  reduced  by  the  formation  of  bone,  by  the  osteoblasts  on  its  wall, 
and  the  size  of  the  root  is  increased  by  the  formation,  layer  after 


CHANGES  IN  PERIDENTAL  MEMBRANE  WITH  AGE        273 

layer,  on  its  surface.  In  this  way  the  thickness  of  the  membrane 
is  reduced.  Figs.  231  and  232  were  made  to  illustrate  this  change. 
They  were  photographed  with  as  nearly  the  same  magnifications 
as  possible,  so  as  to  compare  the  thickness  of  the  layers.  In  the 
first,  there  are  but  two  layers  of  cementum;  notice  the  thickening 
of  the  last-formed  layer,  to  attach  the  strong  bundles  of  fibers  at 
the  angle  of  the  root.  The  second  is  from  a  temporary  tooth  which 
has  been  in  position  and  function  for  a  long  time;  notice  the  thick- 
ness of  the  cementum  and  that  the  formation  of  bone  and  cementum 


Pd 


FIG.  232. — Old  membranes  (from  sheep):  D,  dentin;  Cm,  cementum;  Pd,  peri- 
dental  membrane;  B,  bone  forming  the  wall  of  the  alveolus;  P,  pulp.  (About 
80  X) 


has  reduced  the  thickness  of  the  membrane  to  not  more  than  one- 
third  of  its  original  amount.  Notice  also  that  the  surface  of  both 
bone  and  cementum  are  not  even,  but  scalloped,  and  that  where 
the  cementum  projects  toward  the  alveolar  wall  there!  is  a  depres- 
sion in  the  bone,  and  where  the  bone  projects  toward  the  cementum 
there  is  a  depression  in  the  cementum.  There  is  therefore  a  dis- 
tinct tendency  for  the  two  tissues  to  interlock  but  remain  separated 
by  a  layer  of  fibrous  tissue.  The  author  has  never  seen  a  specimen 
showing  a  union  between  calcified  substances  of  bone  and  cemen- 
18 


274  THE  PERIDENTAL  MEMBRANE 

turn.  Two  surfaces  of  cementum  may  become  united  by  direct 
calcification  and  the  teeth  fused  together.  This  is  illustrated  in 
many  freak  specimens  to  be  found  in  any  dental  museum.  It  is 
often  stated  that  a  tooth  had  become  ankylosed  to  the  bone,  but 
to  the  author's  knowledge  no  specimen  has  ever  been  shown  in 
which  the  separating  layer  of  fibrous  tissue  was  not  present. 

Practical  Consideration. — These  structural  facts  are  of  the  greatest 
practical  importance,  especially  in  the  making  of  gold  fillings  for 
young  persons.  Every  operator  has  noticed  the  greatest  difference 
in  the  feeling  of  the  instrument  under  the  mallet  upon  different 
teeth.  In  one  instance  it  will  ring  under  the  steel  mallet  as  if  the 
tooth  were  resting  on  an  anvil;  in  another  case  it  feels  as  if  the 
tooth  were  resting  on  a  cushion.  In  the  first  case  all  of  the  force 
of  the  blow  is  expended  in  the  condensation  of  the  gold.  In  the 
second,  a  large  proportion  is  lost  in  the  movement  of  the  tooth. 
If  the  membrane  is  thin  and  the  cementum  and  bone  are  inter- 
locked, the  tooth  is  firmly  supported.  If  the  membrane  is  thick 
and  the  fibers  long,  as  in  the  first  illustration,  the  blow  is  dissipated 
in  the  sag  of  the  fibers.  The  tooth  is  jumping  up  and  down  in  its 
socket.  The  force  used  is  dissipated,  the  gold  is  not  condensed, 
and  in  a  very  short  time  an  acute  inflammation  is  set  up  and  the 
tooth  becomes  very  sore  to  the  blows.  This  the  author  believes  is 
the  explanation  of  the  idea  that  gold  wrill  not  preserve  teeth  for 
young  children.  It  has  often  been  said  that  children's  teeth  are 
too  soft  for  gold  fillings.  The  difficulty  is  not  with  the  enamel  and 
dentin,  but  because  of  the  thickness  of  their  membranes.  The 
gold  is  not  sufficiently  condensed  to  exclude  moisture,  and  the 
fillings  fail.  Serious  damage  also  may  be  done  to  the  membrane. 
The  Museum  of  the  Northwestern  University  Dental  School  con- 
tains an  object  lesson  on  this  point.  It  consists  in  a  bicuspid  with 
a  beautifully  condensed  and  finished  gold  filling  in  a  mesial  occlusal 
cavity.  The  history  accompanying  it  is  somewhat  as  follows: 
The  operation  was  undertaken  for  a  patient  aged  about  fourteen 
years.  The  tooth  became  exceedingly  sore  under  the  mallet;  the 
filling  was,  however,  completed  and  polished,  but  a  few  days  later 
the  tooth  was  picked  out  with  the  fingers.  The  peridental  membrane 
had  been  literally  hammered  to  death.  Stated  in  scientific  terms, 
the  fibers  had  sawed  upon  the  bloodvessels,  exciting  an  acute  inflam- 
mation, resulting  in  complete  stasis  and  the  death  of  the  tissue. 
In  all  operations  where  gold  is  to  be  condensed  in  teeth  with  thick 
membranes,  they  must  be  firmly  supported  so  as  to  be  held  rigidly 
against  the  blow. 


CHAPTER  XXII. 

ABSORPTION  OF  TEETH 

BY  NEWTON  G.  THOMAS,  M.A.,  D.D.S. 

THE  absorption  of  teeth  implies  a  phenomenon  which  is  known 
to  occur  in  both  dentitions.  To  the  primary  dentition,  absorption 
is  a  part  of  normal  history;  with  the  permanent  dentition,  it  is 
associated  only  with  unrecognizable  and  inexplicable  conditions  or 
strictly  pathological  agencies.  Hence  our  first  classification  of 
absorption  is  physiological  and  pathological.  Under  the  former 
comes  the  removal  of  temporary  tooth  roots  and  the  roots  of 
implanted  teeth,  while  under  pathological  comes  the  removal  of 
permanent  tooth  roots,  either  wholly  or  in  part.  These  occurrences 
form  the  ground  work  of  this  discussion. 


FIG.  233. — Showing  normal  absorption  of  a  temporary  molar  without  pressure 
from  a  permanent  successor. 

Various  causes  of  the  absorption  of  the  roots  of  temporary  teeth 
have  been  presented.  The  principal  ones  ascribed  are  pressure,  the 
ectodermic  origin  of  enamel,  blood-pressure,  the  gubernaculum 
dentis,  and  the  deposition  of  bone  impelling  the  permanent  teeth 
occlusally.  It  is  urged  that  the  pressure  of  the  erupting  permanent 
teeth  instigates  the  process  whether  one  or  all  of  the  causes  of  erup- 
tion mentioned  begins  or  maintains  the  movement.  It  is  a  common 
observation  that  the  pressure  of  permanent  teeth  sometimes  fails  to 
stimulate  absorption  and  again  it  is  seen  that  temporary  teeth 
often  absorb  when  their  permanent  successors  are  absent  (Fig.  233). 

(275) 


276  ABSORPTION  OF  TEETH 

The  absorption  of  the  roots  of  the  temporary  teeth  must  be  con- 
sidered as  Tomes  considered  it,  a  physiological  or  vital  process, 
which  all  of  the  factors  named  may  abet  but  do  not  explain. 

The  agent  of  absorption  presents  an  interesting  but  unfinished 
study.  Kolliker  designated  the  multinucleated  giant  cell,  the  osteo- 
clast,  almost  always  seen  in  areas  where  osseous  tissue  is  in  process 
of  formation,  the  agent  of  tissue  removal,  because  bone  building  is 
always  a  composite  of  construction  and  destruction,  a  condition 
not  seen  in  tooth  tissues  because  changes  similar  to  those  in  bone 
do  not  take  place.  Once  teeth  are  formed  they  do  not  change  form : 
areas  of  their  surfaces  may  be  removed  but  it  is  never  done  for  the 
purpose  of  reconstruction  as  is  the  case  when  subperiosteal  bone  is 
removed  to  give  place  to  Haversian  system  bone.  Where,  however, 
teeth  are  being  physiologically  removed  osteoclasts  are  found  with- 
out fail.  Until  quite  recently  this  cell  maintained  its  designa- 
tion as  a  tissue  destroyer  undisputed,  but  at  the  present  time 
its  function  is  held  in  question.  It  is  stated  that  there  is  no  positive 
evidence  that  the  osteoclast  is  active  in  destroying  bone.  The 
explanations  given  are  that  they  are  degenerate  osteoblasts  that 
have  become  confluent,1  or  that  have  been  affected  by  the  agency 
that  is  affecting  the  bone,  and,  therefore,  are  in  a  condition  of 
degeneracy.2  Also  destruction  of  bone  by  halisteresis  as  in  osteo- 
malacia  is  mentioned  to  prove  that  cells  are  not  necessary  to  its 
accomplishment.3  Both  of  these  hypotheses  are  unsatisfactory. 
Sections  can  be  readily  produced  in  which  osteoclasts  show  no 
evidence  of  disintegration  and  in  which  there  is  no  evidence  that 
they  are  the  products  of  osteoblastic  fusion.  In  fact  cementoblasts, 
the  analogue  of  osteoblasts,  are  noticeably  absent  from  the  surfaces 
of  teeth  on  which  absorption  is  in  progress. 

The  origin  of  osteoclasts  is  controverted  as  much  as  their  function. 
Prentiss,  Jackson  and  Dautschakoff  state  that  they  are  derived  from 
the  reticular  cells  of  marrow.  Kolliker,  Bredichin,  and  Howell 
suggest  that  they  are  osteoblastic  in  origin,  the  osteoblast  in  response 
to  some  unknown  stimulus  fusing  with  its  fellows  to  assume  a  new 
function.  \Yegener,  Schaffer,4  Fischer,5  and  Mallory6  trace  them  to 

1  Arey:  The  Origin,  Fate  and  Significance  of  Osteoclasts,  Tr.  Chicago  Path.  Soc. , 
pp.  231-234. 

2  Stohr:     Text-book  of  Histology,  pp.  68  and  202. 

3  Loc.  cit. 

4  Prentiss:     Origin  and  Fate  of  Osteoclasts,  Surg.,  Gynec.  and  Obstet.,  1915,  xx, 
678. 

*  Anatomische  Hefte,  1909. 

6  Principles  of  Pathology,  1912,  p.  52. 


ABSORPTION  OF  TEETH  277 

endothelial  cells,  while  Ranvier,  Duval,  and  Bohm1  assert  that  they 
arise  from  lymphoid  marrow  cells.  In  the  consideration  of  the 
internal  absorptions  seen  in  dentin,  Causch2  avers  that  they  are  the 
products  of  odontoblasts,  as  does  Salter  also.  To  this  Causch  adds 
that  tissue  destruction  does  not  depend  upon  them,  as  phagocytic 
leukocytes  may  produce  the  same  results.  In  this  it  will  be  observed 
he  harmonizes  closely  with  Mallory.  The  specimens  from  which 
the  accompanying  figures  are  made  also  testify  to  the  endothelial 
origin  of  the  osteoclast.  Fischer's  assumption  that  the  endothelium 
of  capillaries  may  destroy  tissue  is  also  closely  related  to  Mallory's 
statement.3  Bland-Sutton,4  in  1887,  described  giant  cells  formed  by 
the  fusion  of  phagocytes,  and  adds:  "The  large  multinuclear  osteo- 
clasts  seen  in  places  where  vertebrate  bone  and  teeth  are  under 
absorption  must  also  be  placed  in  the  same  category.''  In  accord 
with  the  foregoing,  Mallory  says :  "  When  an  endothelial  leukocyte 
finds  difficulty  in  dissolving  a  substance,  as,  for  instance,  lime  or 
certain  fat  products  or  the  blastomyces,  it  frequently  fuses  with 
other  endothelial  leukocytes  to  form  a  multinucleated  mass  of  cyto- 
plasm commonly  termed  a  foreign  body  giant  cell.  If  the  foreign 
body  is  too  large  for  one  leukocyte  to  incorporate  (cholesterin  crystals, 
hairs)  one  or  more  giant  cells  are  formed  which  surround  it  or  plaster 
themselves  to  its  surface."5  With  this  Delafield  and  Pruden  agree.6 
It  is  quite  certain  that  the  osteoclast  is  not  the  product  of  mitosis, 
as  Kolliker  thought,  without  cytoplasmic  division  as  mitotic  figures 
are  never  seen  in  it. 

In  the  sections  which  formed  the  basis  of  this  study  irregular 
foveolse  are  seen  in  which  are  groups  of  cells  apparently  of  leuko- 
cytic  origin,  which  to  all  appearances  are  active  in  the  removal  of 
calcified  tissue,  while  on  the  surface  continuous  with  that  on  which 
they  work,  are  multinucleated  cells  in  great  numbers  filling  smoothly 
formed  spaces.  This  condition  may  be  explained  by  the  interpreta- 
tion of  Button,  Causch  and  Mallory.  The  phagocytic  endothelial 
cells  introduce  the  process  of  tissue  removal  and  later  fuse,  according 
to  Mallory's  hypothesis,  continuing  the  destruction  of  tissues  after 
their  fusion,  thus  forming  the  smooth  bay-like  excavations  noted. 
This  also  explains  the  low  number  of  giant  cells  found  in  the  early 

1  Loc.  cit. 

2  Transactions  of  World's  Columbian  Dental  Congress,  p.  114. 

3  Mallory:    Principles  of  Pathology,  1912,  p.  52. 

4  Introduction  to  General  Pathology,  1887,  p.  124. 
&  Mallory:     Principles  of  Pathology,  1912,  p.  52. 
6  Text-book  of  Pathology,  p.  119. 


278 


ABSORPTION  OF  TEETH 


stages  of  endochondral  bone  formation,  which  point  is  mentioned 
by  Stohr1  and  emphasized  by  others.  Phagocytic  endothelial  cells 
or  other  means  of  calcified  tissue  removal  may  be  employed.  To 
the  writer  it  seems  conclusive  that  Mallory's  assumption  is  correct. 
The  procedure  of  tissue  removal  is  difficult  of  explanation.  To  the 
present  time  it  has  not  been  determined  that  osteoclasts  or  phago- 
cytic  leukocytes  produce  acid  for  this  purpose.  Also  the  process  is 
more  than  decalcification.  In  decalcification  we  know  that  certain 
tissues  resist  acid  for  varying  periods  of  time.  In  the  process  under 
discussion  we  have  complete  tissue  removal,  the  connective-tissue 
matrix  of  the  calcified  structures  and  the  dense  peridental  membrane 


_  -'  —  *f^t*£s~/!': 


**'•••&       "*  ^:  ^  v-.\  v        r 

~  — '-A-ftJS*-  --_A^,_'_V-V->.  ,  -  _  -  C  . 


FIG.  234. — Section  showing  absorption  of  the  tooth  of  a  sheep:  a,  cementum ;    fo, 
osteoclasts  in  cementum  and  den  tin;  c,  osteoclast  in  the  peridental  membrane. 

as  well  (see  234,  c).  In  connection  with  the  endochondral  bone 
formation  it  has  been  suggested  that  reduction  of  blood  supply 
causes  autolysis  of  cells  in  the  cartilaginous  matrix  and  a  consequent 
dissolution  of  the  calcified  cartilage  spicules  by  the  enzymes  set 
free.  In  the  light  of  the  foregoing  the  last  hypothesis  seems  unneces- 
sary. It  is  commonly  accepted  that  osteoblasts  may  become  osteo- 
clasts because  it  is  known  that  cells  long  inactive  may  change  their 
function,  or  that  connective-tissue  cells,  under  changed  conditions, 
may  develop  specializations,  or  cells  long  inactive  may  resume  func- 
tional activities  of  a  different  character  from  that  carried  on  in 


1  Text-book  of  Pathology,  pp.   68  and  202. 


ABSORPTION  OF  TEETH  279 

their  earlier  histories.  Thus  liberated  cartilage  or  bone  corpuscles 
may  become  cartilage  or  bone  builders.  By  injecting  lamp-black 
or  bacteria  into  the  subarachnoid  space  Weed1  found  that  connective- 
tissue  cells  became  phagocytic  and  ameboid.  Hassin2  found  that  glia 
cells  did  similarly,  devouring  myelin  and  migrating  to  the  vessels 
of  the  area  as  did  Nissl  and  Altzheimer.3  Similar  phenomena  have 
been  observed  by  various  workers  on  other  tissues  and  organs. 

At  birth  the  jaw  contains  all  the  deciduous  teeth  and  likewise  the 
germs  of  the  permanent  teeth  except  the  second  and  third  molars. 
Three  to  five  years  are  required  for  the  completion  of  the  roots  after 
which  they  remain  complete  for  a  similar  length  of  time.  During 
this  period  the  permanent  teeth  have  been  developing  in  their 
crypts  after  which  they  begin  their  occlusal  movement.  The  first 
observation  of  importance  is  the  appearance  of  osteoclasts  on  the 
roof  of  the  crypt.  Penetrating  the  crypt  roof  the  permanent  tooth 
approaches  the  lingual  surface  of  the  temporary  tooth  if  it  is  an 
incisor  or  cuspid,  and  immediately  between  the  roots  if  it  be  a  pos- 
terior tooth.  Incisors  of  dogs  have  a  tendency  to  point  directly  to 
the  apices  of  their  temporary  predecessors  (see  Fig.  235)  while 
those  of  sheep  simulate  those  in  the  human  mouth,  approaching 
the  lingual  surface.  The  difference  presents  interesting  features  for 
our  notice.  The  removal  of  the  tissue  in  the  path  of  the  advancing 
tooth  is  more  rapid  than  the  advance  of  that  tooth  with  the  result 
that  the  way  cleared  is  filled  with  young  fibrous  connective  tissue 
rich  in  budding  capillaries  (see  Fig.  234,  d).  In  the  wake  of  the 
tooth,  bone  spicules  are  developed  supportive  to  the  crypt  for  it  will 
be  observed  that  at  first  the  crypt  moves  with  the  structures  it 
contains,  thus  affording  an  important  mechanical  factor  in  the 
development  of  the  jaw. 

Coincident  with  the  approach  of  the  permanent  tooth  germ  to  the 
root  of  the  temporary  tooth  osteoclasts  appear  on  the  approached 
surface  of  the  deciduous  root.  Also  capillary  loops  develop  extending 
toward  them  in  a  manner  strikingly  similar  to  that  seen  when  calcifi- 
cation is  in  progress  for  these  activities  always  call  for  a  copious 
blood  supply  (see Figs.  235,  a,  and  237, 6).  The  work  of  the  osteoclast 
is  never  long  confined  to  the  area  mentioned.  The  stimulus  afforded 

1  The  Establishment  of  the  Circulation  of  the  Cerebrospinal  Fluid,  Anat.  Record, 
x,    256-158. 

2  Histopathological  Changes  in  a  Case  of  Amyotrophic  Lateral  Sclerosis,   Med. 
Rec.,  February  10,   1917. 

3  Histologische   und    Histopathologische,    Arbeiten    von    Nissl    und    Antzheimer, 
1912. 


280 


ABSORPTION  OF  TEETH 


to  the  peridental  membrane  soon  permeates  it  with  the  result  that 
osteoclasts  appear  on  any  surface,  anywhere  from  the  apex  to  the 
gingival  line  (see  Fig.  235,  6).  Occasionally  the  spaces  excavated 
are  filled  with  cementum  and  a  new  attachment  made,  but  that  is 
far  from  consistent.  Whereas  the  approaching  permanent  tooth 
apparently  is  the  original  stimulus  to  the  destructive  process  the 


' 


FIG.  235. — Section  of  a  dog's  tooth,  showing  internal  and  external  absorption 
A,  capillary  loops  to  absorption  areas;  B,  absorption  area  at  the  cervix  of  the  tooth; 
C,  foveolse  on  pulpal  surface  of  root;  D,  cellular  layer  surrounding  the  pulp. 

ragged  edges  made  by  the  absorption  doubtless  afford  a  secondary 
stimulus  and  the  area  where  no  new  cementum  is  deposited  has  the 
effect  of  a  foreign  body,  all  of  which  tends  to  speed  the  absorption. 
The  beginning  of  absorption  and  its  continuance  are  in  no  way 
affected  seemingly  by  the  fact  of  pulp  extirpation  provided  that  the 


ABSORPTION  OF  TEETH  281 

root  is  aseptic.  Instances  are  plentiful  showing  perfect  removal  of 
the  root  leaving  a  cone  of  filling  material  in  the  tissue.  A  curious 
and  interesting  phenomenon  may  be  observed  in  the  museum  of  the 
Northwestern  University  Dental  School.  A  series  of  teeth  are 
shown  on  which  is  a  small  tube  of  dentin  around  the  canal  which 
was  preserved  around  the  pulp  apparently  in  resistance  to  the  absorb- 
ing agents.  Normally  the  process  continues  until  the  root  is  wholly 
gone  and  it  is  often  seen  that  the  dentin  of  the  crown  has  been 
entirely  removed  and  sometimes  the  enamel  has  been  reduced  to  the 
thinness  of  tissue  paper. 

Apparently  no  changes  appear  in  the  pulp  due  to  the  external 
absorption  of  the  tooth  root.  The  writer  has  never  observed  any 
effects  upon  the  pulp  due  to  changes  in  progress  on  the  exterior  of 
the  tooth  root;  it  shows  no  reaction  until  it  is  invaded.  The  embry- 
onic character  of  the  tissue  naturally  undergoes  immediate  alteration 
when  its  environment  is  changed.  When  the  absorption  has  been 
greatest  at  the  apex  and  a  large  area  of  pulp  is  uncovered  the  effect 
upon  the  pulp  is  widespread.  Around  the  periphery  new  connective- 
tissue  elements  appear  extending  farther  and  farther  occlusally 
around  the  pulpal  walls  until  all  the  odontoblasts  are  lost  and  in 
their  places  is  a  dense  cellular  zone  containing  a  preponderance  of 
undifferentiated  connective-tissue  cells  (see  Fig.  235,  d).  Upon 
the  outer  surface  of  this  cellular  layer  osteoclasts  appear  and  inter- 
nal absorption  accompanies  that  which  progresses  on  the  exterior 
root  surface  (see  Fig.  235,  c).  Some  fibroblasts  are  seen  and  an 
abundance  of  capillary  loops  extends  radially  from  much  enlarged 
central  vessels  to  the  absorbing  cells  (see  Fig.  235,  a).  Hence,  the 
pulp  has  been  metamorphosed  into  a  scrap  of  typical  granulation 
tissue.  Should  the  opening  into  the  pulp  chamber  elsewhere  be 
small,  similar  changes  occur  in  the  immediate  vicinity  of  the  penetra- 
tion. The  more  distant  parts  of  the  pulp,  be  they  coronal  or  apical, 
remain  practically  normal  until  the  point  of  invasion  has  become 
large  enough  to  affect  the  entire'structure  or  numerous  penetrations 
are  made. 

During  this  process  it  will  be  noted  that  although  pressure  may 
be  assigned  as  the  stimulus  to  absorption  that  stimulus  is  never 
retroactive.  No  osteoclasts  ever  appear  inside  the  follicle  of  the 
erupting  tooth  which  causes  the  pressure.  Also,  should  acid  be 
produced  by  the  cells  for  the  purpose  of  decalcification  it  never 
affects  the  permanent  tooth.  The  follicle  seems  to  be  a  sufficient 
protection  against  such  emergencies,  and  it  persists  until  the  tooth 


282 


ABSORPTION  OF  TEETH 


reaches  the  surface  of  the  gum.  It  may  likewise  be  inferred  that  no 
tissue  can  be  referred  to. as  an  absorbent  organ,  as  we  have  seen  that 
absorption  extends  over  the  surfaces  of  the  tooth  externally  as  well 
as  internally.  Absorption  of  the  roots  of  the  teeth  of  different 
species  is  observed  to  follow  a  routine  which  is  a  modification  of 
the  one  described,  the  general  principle  being  the  same. 

Under  the  head  of  physiological  absorptions  must  be  considered 
the  removal  of  implanted  teeth.  It  has  been  long  observed  that 
implanted  teeth  are  of  brief  service  in  the  mouth  and  that  when  they 
are  removed  their  root  surfaces  are  pitted  and  rough  or  entirely 
absorbed  (Figs  236  and  241).  Although  to  my  knowledge  no 
sections  of  implanted  teeth  have  ever  been  made  with  the  sur- 
rounding supporting  tissue  the  explanation  of  both  their  short  period 
of  serviceability  and  the  pitted  surfaces  seems  obvious.  The 


FIG.  236. — A  bicuspid    tooth  which  was   implanted  and    remained  in    the    alveolus 
about  three  years.      (Fig.    119  in  Special  Dental  Pathology,  Black.) 

inserted  tooth  is  placed  in  an  artificially  created  alveolus  which 
nature  attempts  to  close.  To  do  so  agents  for  the  removal  of  the 
foreign  body  attack  its  surface  and  bone  formation  follows  in  the 
wake  filling  the  indentations  with  its  extensions.  The  attack  in  this 
case  is  uniform  upon  the  surface  of  the  root  unless  there  are  patho- 
logical interferences.  A  great  surgeon  is  accredited  with  saying 
"The  more  perfect  the  operation  of  placing  the  tooth  the  more  rapid 
is  the  removal"  (Gilmer).  It  is  the  projections  of  bone  into  the 
foveolse  made  by  the  osteoclasts  that  give  the  tooth  its  firmness. 
An  a:-ray  of  such  a  tooth  shows  no  clear  periphery  as  is  the  case  with 
teeth  normally  attached  but  rather  a  confused  picture  due  to  the 
bridges  of  bone  extending  into  the  tooth  root. 

Under  pathological  absorptions,  first  come  those  found  in  the 
walls  of  the  pulp  chamber.     Causch1  mentions  excavations  in  the 

1  Tr.  of  World's  Columbian  Dental  Congress,  p.  114. 


ABSORPTION  OF  TEETH 


283 


pulpal  walls  as  does  Salter  and  describes  the  same  as  filled  with 
bone.1  It  will  be  remembered  that  the  older  histologists  and  some 
modern  ones  call  every  tooth  tissue  bone,  if  it  is  not  definite  in 
structure.  It  was  his  findings  in  these  studies  that  led  him  to  con- 
sider odontoblasts  as  contributing  to  the  formation  of  osteoclasts. 
Absorptions  in  the  dentin  surrounding  the  pulp  chamber  and  canals 


FIG.  237. — Section  of  a  dog's  tooth,  showing  blood  supply  to  enamel  forming  cells. 
A,  ameloblasts;  B,  vessels. 

are  very  common  and  not  infrequently  contain  filling  of  calcified 
material  varying  in  structure  from  an  irregularly  arranged  dentin 
to  a  clear  structureless  deposit  (Figs.  239  and  240).  No  one  has 
observed  osteoclasts  in  the  pulp-chamber  of  a  tooth  that  has  not 
been  invaded.  But  there  is  no  reason  for  doubting  that  they  may 
appear  there  and  other  phagocytes  as  before  mentioned  may 


Black:     Special  Dental  Pathology,  p.  265. 


284 


ABSORPTION  OF  TEETH 


accomplish  the  results  observed.  The  observations  are  there  and 
frequently  enough  penetrate  to  the  outside.  Hess  in  a  series  of 
studies  on  multiple  foramina  reports  that  canals  are  often  formed 
from  within  out  to  compensate  for  canals  closed  by  secondary 
deposits  of  cementum.1  Such  fillings  of  canals  are  common  obser- 
vations in  ground  sections  (see  Fig.  131). 

Absorptions  on  permanent  teeth  are  very  common.  They  are 
associated  with  impactions  and  are  noticed  on  the  apices  of  roots 
about  which  are  abscesses  as  well  as  around  the  cervices  of  teeth. 
Sometimes  the  abscess  is  given  as  the  possible  cause  of  the  tissue 


FIG.  238. — Photograph  of  section  from  which  Fig.  237  was  made. 

destruction.  It  does  not  seem  probable,  however,  that  the  acid  con- 
tent of  pus  destroys  the  tooth  root  and  it  is  very  certain  that  no  cell, 
osteoblast  or  osteoclast,  ever  approaches  a  root  which  has  been 
bathed  in  pus  as  it  does  under  physiological  conditions.  Believing 
that  the  tissue  destruction  is  accomplished  by  cells  and  not  by 
acids,  the  excavations  must  be  made  before  the  pus  reaches  the 
cementum,  the  cells  being  stimulated  to  activity  possibly  by  the 


1  Hess:     The  Development  and  Structure  of  the  Tooth  Apex  and  Features  Per- 
taining Thereto,  Zahnheilkunde,   1917,  xxxvi. 


ABSORPTION  OF  TEETH 


285 


inflammation.  Explaining  the  other  absorptions  mentioned  Inglis 
suggests  that  such  causes  as  protruding  root  canal  fillings,  broaches, 
pericemental  deposits  and  salivary  calculus  may  instigate  cellular 
activity.1  It  is  true  that  the  absorptions  occur  most  commonly 


FIG.  239. — Section  of  human  tooth,  showing  an  internal  absorption  area  which 
has  been  almost  completely  filled  with  structureless  calcified  material.  A,  primary 
dentin;  B,  foveolse;  C,  structureless  calcified  material;  D,  root  canal. 

on  the  cervical  and  apical  areas  where  inflammations  are  the  com- 
monest (see  Fig.  241). 

More  interesting  than  the  foregoing  is  the  entire  removal  of  the 
roots  of  permanent  teeth,  sometimes  limited  to  a  single  tooth,  or,  as 


1  Burchard  and  Inglis:     Dental  Pathology  and  Therapeutics,   1912,  p.  622. 


286 


ABSORPTION  OF  TEETH 


has  been  reported  by  Black,1  of  all  the  teeth,  in  exactly  the  same  way 
as  deciduous  teeth  are  removed.    Where  such  removals  have  taken 


FIG.  240. — Showing  absorption  of  pulpal  walls  and   newly  deposited,  structureless, 
calcined  tissue.     A,  dentin;  B,  foveolae;  C,  new  calcified  tissue;  D,  canal. 


FIG.  241. — Showing  absorption  of  a  tooth  implanted  by  Dr.  Thomas  L  Gilmer. 
When  this  radiograph  was  taken  the  tooth  had  been  in  the  alveolus  nearly  three 
years.  (Fig.  118,  Special  Dental  Pathology,  Black). 


Special  Dental  Pathology,  p.  33. 


ABSORPTION  OF  TEETH  287 

place  the  patient  has  never  reported  any  accompanying  symptoms. 
The  process  has  been  painless.  No  etiology  of  such  conditions  is 
forthcoming.  That  question  being  laid  aside  there  is  no  reason  for 
doubting  that  the  agents  employed  are  the  same  as  for  deciduous 
teeth.  Could  sections  be  made  of  these  teeth  in  situ  doubtless  we 
should  find  upon  their  surf  aces  osteoclasts  accomplishing  the  purpose. 
To  summarize  it  seems  strongly  evident  that  osteoclasts  or  their 
endothelial  predecessors  are  the  active  agents  of  absorption,  although 
the  method  by  which  they  accomplish  it  is  unknown.  Such  is  their 
distribution  on  both  the  internal  and  external  surfaces  of  the  tooth 
that  neither  the  pulp  nor  the  peridental  membrane  can  logically  be 
termed  an  absorbent  organ.  These  cells  destroy  soft  and  hard 
tissues  alike,  outlines  of  them  being  visible  in  the  dense  peridental 
membrane  surrounding  the  wasting  tooth  (see  Fig.  234,  c).  Especial 
emphasis  is  laid  upon  the  connective-tissue  changes  that  take  place, 
changes  in  both  the  hard  and  soft  tissues  as  well  as  changes  in  the 
blood  supply.  What  seems  so  evident  in  the  study  of  the  removal 
of  the  temporary  teeth  and  in  bone  seems  a  well  justified  explanation 
of  the  removal  of  the  structures  mentioned  where  exact  data  is 
so  difficult  to  acquire. 


CHAPTER  XXIII. 
THE  MOUTH  CAVITY. 

Mucous  Membrane. — The  mucous  membrane  lining  the  mouth 
cavity  is  composed  of  a  layer  of  stratified  squamous  epithelium 
supported  upon  a  tunica  propria,  which  is  usually  described  as 
composed  of  two  parts — the  papillary  layer  and  the  reticular  layer. 
The  epithelium  and  the  tunica  propria  make  up  the  mucous  mem- 
brane proper,  which  is  supported  upon  a  submucous  layer  com- 
posed of  a  coarse  network  of  white  and  elastic  fibers,  containing 
the  larger  bloodvessels. 

The  Epithelium. — The  stratified  squamous  epithelium  is  provided 
with  a  horny  or  corneous  layer  only  in  the  portions  covering  the 
alveolar  process  and  the  hard  palate,  or,  in  other  words,  where  the 
submucosa  is  firmly  attached  to  the  periosteum  (Fig.  242).  In 
these  positions  the  horny  layer  consists  of  dead  cells  which  have 
lost  their  nuclei  and  whose  cytoplasm  has  been  converted  into 
keratin  or  horny  material. 

These  scale-like  cell  remains  are  closely  packed  into  a  protec- 
tive layer.  There  is  no  distinct  stratum  lucidum  separating  the 
dead  from  the  living  cells,  as  there  is  in  the  skin.  In  the  deeper 
portions  the  cells  possess  oval  or  rounded  nuclei  and  become  larger 
and  more  polyhedral  as  the  basemerft  membrane  is  approached. 
The  cells  of  the  deepest  layer  next  to  the  basement  membrane  are 
tall  and  approach  the  columnar  form,  but  are  never  much  greater 
in  height  than  width.  The  deep  layer  is  often  called  stratum  Mal- 
pighii.  The  epithelium  lining  the  gingival  space  and  that  covering 
unattached  portions  is  without  the  horny  layer,  and  the  cells  are 
larger  and  more  loosely  placed.  The  polyhedral  cells  in  the  middle 
portion  of  the  layer  show  distinct  intercellular  spaces  across  which 
the  cytoplasm  extends  in  intercellular  bridges. 

Isolated  cells  from  this  region  show  the  broken  bridges  project- 
in'g  from  their  surface,  and  for  this  reason  have  been  called  "  pickle 
or  prickle  cells."  In  these  positions  the  thickness  of  the  epithelial 
layer  is  usually  greater  than  in  the  attached  portions  of  the  mem- 
brane (Fig.  243). 
(288) 


SUBMUCOSA 


289 


Tunica  Propria. — The  connective-tissue  layer  of  the  mucous 
membrane  interlocks  with  the  epithelial  layer  by  means  of  the 
tunica  papillaris,  which  is  composed  of  very  delicate  white  and 
elastic  connective-tissue  fibers.  They  are  usually  about  half  as 
tall  as  the  thickness  of  the  epithelium,  and  about  one-third  as  wide 
as  they  are  tall.  The  height  and  character  of  the  papillae  varies 
greatly,  however,  in  different  position.  In  the  red  border  of  the 
lip  and  in  the  epithelium  lining  the  gingival  space  they  are  very 
tall  and  narrow,  and  approach  very  close  to  the  surface  of  the 
epithelium.  Over  the  gums  and  the  palate  they  are  much  shorter 


FIG.  242. — Stratified  squamous  epithelium  covering  the  alveolar  process:     C,  cor- 
neous layer;    P,  papilla  of  connective  tissue.     (About  400  X) 

and  wider  and  do  not  extend  more  than  half-way  through  the 
epithelium.  These  papillae  contain  loops  of  capillary  bloodvessels 
and  in  some  special  nerve  endings  are  found. 

Reticular  Layer. — The  reticular  layer  joins  the  papillary  layer 
without  any  line  of  demarcation,  and  is  composed  of  the  same  kind 
of  tissue,  the  fibers  being  arranged  in  a  delicate  network.  Every- 
where in  the  tunica  propria  are  found  ducts  from  mucous  gland 
which  lie  in  the  deeper  layers. 

Submucosa. — The  submucosa  is  composed  of  firm  connective 
tissue  in  which  the  white  fibers  are  in  large,  strong  bundles,  and 
19 


290 


THE  MOUTH  CAVITY 


elastic  fibers  are  scarce.  It  contains  two  plexuses  of  bloodvessels, 
both  more  or  less  parallel  with  the  surface.  The  outer  is  composed 
of  small  vessels  forming  a  small-meshed  network,  the  deeper  of 
large  vessels  more  widely  separated.  Lymphatic  vessels  everywhere 
follow  the  course  of  the  bloodvessels. 

Glands  of  the  Submucosa. — The  submucosa  contains  a  great  many 
small  tubular  glands.  These  are  distributed  widely  over  the  tongue 
and  membrane  of  the  cheek  and  lip  (Fig.  244).  They  are  branched 
tubular  glands,  sometimes  simple  and  sometimes  compound. 
The  body  of  the  gland  is  always  in  the  submucosa,  though  it  may 
extend  into  the  underlying  muscle.  Some  are  serous  and  others 


FIG.  243. — Stratified  squamous  epithelium  from  unattached  mucous  membrane  of 
the  mouth.     The  corneous  layer  is  absent.     (About  200  X) 

mucous,  while  many  of  the  larger  ones  contain  cells  of  both  types. 
The  secretion  of  these  glands  is  probably  much  more  important 
than  has  been  supposed. 

Nerve  Endings  in  the  Mucosa. — Sensory  nerve  endings  of  two 
kinds  are  found  in  the  mucous  membrane.  Krause's  end  bulbs 
are  found  in  many  of  the  papillae,  and  other  nerves  terminate  in 
free  endings  lying  between  the  epithelial  cells. 

The  Tongue. — The  tongue  is  composed  of  a  mass  of  voluntary 
muscle  fibers  arranged  in  complicated  interlacing  bundles,  covered 
by  the  mucous  membrane.  The  most  striking  characteristics  of 
the  mucous  membrane  of  the  tongue  (Fig.  245)  are:  (1)  The 
thinness  of  the  submucosa,  which  holds  it  closelv  to  the  mass  of 


THE  MUSCLES 


291 


muscle  and  allows  very  little  movement  of  it;  (2)  the  submucosa 
in  the  dorsal  surface  contains  no  glands,  though  there  are  glands 
among  the  muscle  fibers  whose  ducts  pass  through  the  submucosa; 
(3)  the  presence  of  the  epithelial  papilte  upon  its  dorsal  surface. 


Mncous 
gland' 


Epithelium 

of  mucous 
membrane 


•\Hair 
follicles 


Epidermis 


Epithelium 

of 
membrane 

'ross  sections 


Longitudinal 

sections 
of  muscle 
fibres 


MUCOUS 

membrane 

with  hii/h 

papillie. 


FIG.  244. — SectioD  through  the  upper  lip  of  a  two-and-a-half-y ear-old  child. 
(14  X)     (Szymonowicz.) 

The  tongue  is  imperfectly  divided  vertically  on  the  median  line 
by  a  band  of  connective  tissue  forming  the  median  raphe  or  septum, 
which  causes  the  depression  at  the  central  line  of  the  dorsal  surface. 
The  Muscles. — The  muscles  of  the  tongue  include  two  groups — 
the  extrinsic  and  the  intrinsic.  The  extrinsic  muscles  comprise 


292 


THE  MOUTH  CAVITY 


the  genioglossus,  the  hyoglossus,  the  styloglossus,  and  the  palato- 
glossus.  These  are  all  paired  and  extend  from  the  skull  or  the  hyoid 
bone  into  the  tongue.  The  intrinsic  muscles  comprise  the  principal 
muscles  of  the  tongue,  the  lingualis.  A  transverse  section  through 
the  body  of  the  tongue  in  the  central  portion  shows  a  complicated 
network  of  muscle  fibers  running  in  three  directions — longitudinally, 
transversely,  and  vertically.  The  longitudinal  fibers  are  arranged 
around  the  outer  portion,  forming  a  cortical  layer  about  5  mm. 
thick.  These  constitute  the  chief  bulk  of  the  lingualis,  supple- 


FIG.  245. — A  section  from  the  side  of  the  tongue:     E,  epithelium;    Sm,  submucosa; 
Bv,  bloodvessels;  M,  muscle  fibers;  G,  mucous  glands. 


mented  by  fibers  from  the  styloglossus.  The  vertical  fibers  are 
mostly  deeply  placed  in  the  central  portion  on  either  side  of  the 
raphe.  They  are  chiefly  derived  from  the  genioglossus  and  radiate 
toward  the  dorsal  surface.  The  transverse  fibers  are  entirely  from 
the  lingualis  except  for  a  few  from  the  palatoglossus.  They  arise 
from  the  septum  and  interlace  with  the  longitudinal  and  vertical 
fibers.  They  break  up  into  strands  running  between  the  longitu- 
dinal fibers  of  the  cortical  portion,  and  spread  out  to  a  submucous 
insertion. 


THE  PAPILLA 


293 


The  complicated  movements  of  the  tongue  are  accomplished 
by  the  contractions  of  these  sets  of  muscles.  When  the  longitudinal 
fibers  are  relaxed  and  the  transverse  fibers  contracted  the  tongue 
is  rolled  and  extended.  When  the  transverse  fibers  are  relaxed 
and  the  vertical  fibers  contracted  the  tongue  is  flattened.  The 
division  of  the  tongue  on  the  median  line  by  the  septum  allows 
each  half  to  work  independently,  so  that  when  the  longitudinal 
fibers  are  contracted  on  one  side  and  relaxed  on  the  other  the  tip 
of  the  tongue  is  moved  sidewise. 


Fio.  246. — Mucous  membrane  from  the  dorsal  surface  of  the  tongue  of  a  kitten, 
showing  filiform  and  f ungiform  papillae. 

The  Papillae. — The  roughness  of  the  dorsal  surface  of  the  tongue 
is  caused  by  projections  of  the  epithelium  resting  upon  the  tunica 
propria,  forming  the  papillae  of  the  tongue.  These  projections  are 
not  to  be  confused  with  the  connective-tissue  papillae  in  the  tunica 
propria  of  the  mucous  membrane.  They  are  of  three  kinds — the 
filiform  and  fungiform  papillae,  which  are  found  over  the  entire 
dorsal  surface,  and  the  circumvallate  papillae,  which  are  limited 
in  number  and  confined  to  the  posterior  portion.  The  filiform  are 
much  the  more  numerous,  especially  near  the  tip  of  the  tongue. 
They  are  from  0.5  to  2.5  mm.  in  height,  and  often  end  in  brush-like 
strands  of  epithelial  cells. 


294 


THE  MOUTH  CAVITY 


The  fungiform  papillae  form  the  red  points  on  the  surface  of  the 
tongue,  especially  near  the  edges,  because  of  the  thinness  of  their 


FIG.  247. — Mucous  membrane  from  the  tongue  of  a  rabbit,  showing  circumvallate 
papillae,  with  taste-buds  on  their  sides. 

epithelium.  They  are  low  and  rounded  in  form,  from  0.5  to  1.5  mm. 
in  height,  and  are  named  from  their  mushroom-like  appearance. 
Fig.  246,  a  section  from  the  tongue  of  a  kitten,  shows  the  form  of 
both  of  these  papilhe.  The  circumvallate  papillae  usually  number 


FIG.  248. — A  section  of  a  taste-bud:    p,  pore;   g,  gustatory  cells;    ep,  epithelial 
cells;   s,  sustentacular  cells;    h,  bristles  of  the  gustatory  cells.     (Schaefer.) 

nine  or  ten,  and  are  arranged  in  a  V-shaped  form  near  the  base  of 
the  tongue,  with  the  apex  extending  backward.    They  are  from  1 


THE   TASTE-BUDS 


295 


to  1.5  mm.  in  height  and  from  2  to  3.5  mm.  in  width.  They  are 
surrounded  by  a  depression,  so  that  the  upper  surface  of  the  papillae 
is  not  much  above  the  general  level  of  the  membrane. 

The  Taste-buds. — These  are  found  chiefly  on  the  sides  of  the 
circumvallate  papillae  (Fig.  247),  though  they  are  occasionally 
found  in  the  epithelium  of  the  fungiform  papilla?  and  the  soft  palate, 
and  on  the  posterior  surface  of  the  epiglottis.  They  are  always 
entirely  embedded  in  the  epithelium  and  extend  through  its  entire 
thickness.  The  structures  are  ovoid  in  form,  with  the  rounded 
end  toward  the  connective  tissue  and  the  pointed  end  at  the  sur- 


Epithe- 
lium 


Tunica 
propria 
Lymph 
nodule- 


Oblique 

sfdion 

of  duct — 

of  mucous 

qland 

Muscle 

fibres 
cut — £ 

trans- 
versely 

FIG.  249. — Section  through  a  lingual  follicle  in  man:    x,  crypt.     (50  X) 
(Szymonowicz.) 


face,  where  a  small  opening,  the  taste-pore,  communicates  with 
the  mouth  cavity  (Fig.  248).  Most  of  the  cells  are  elongated  and 
spindle-shaped,  and  arranged  like  the  leaves  of  an  onion.  Four 
varieties  may  be  recognized.  The  outer  sustentacular  cells  form 
the  outer  layer  and  are  in  contact  with  the  epithelial  cells.  They 
are  elongated,  with  an  oval  nucleus  near  the  center.  The  inner 
sustentacular  are  rod-shaped  cells,  more  slender  in  form,  with  a 
nucleus  at  the  base.  The  neuro-epithelial  cells  are  elongated, 
spindle-shaped  cells  at  the  center  of  the  taste-bud.  The  nucleus 
is  at  the  base  of  the  cell,  and  from  the  opposite  end  a  stiff  bristle- 
like  process  extends  through  the  taste-pore. 


296 


THE  MOUTH  CAVITY 


The  basal  cells  are  irregular  in  form  with  large  oval  nuclei;  they 
communicate  with  each  other  and  the  sustentacular  cells  by  cyto- 
plasmic  bridges.  They  form  the  base  of  the  taste-bud.  The  func- 


Epithelium 
of  pharynx~~-gf 


Mucous  glands*1" 


Blood  vessel 


Connective-tissue 
capsule 


FIG.  250. — Section  through  a  dog's  tonsil.    At  x,  x  there  are  seen  leukocytes 
which  have  wandered  out  from  the  follicles.      (15  X)     (Szymonowicz.) 


tion  of  the  taste-buds  is  probably  related  to  the  function  of  degluti- 
tion rather  than  the  sensation  of  taste. 

The  Tonsil.- — In  the  posterior  part  of  the  tongue  and  the  wall 
of  the  pharynx  is  found  adenoid  tissue  in  the  form  of  solitary  follicles 


THE  TONSIL  297 

lying  in  the  tunica  propria  and  invading  the  epithelium.  This 
adenoid  tissue  forms  an  organ  which  Waldeyer  has  called  the  lym- 
phatic pharyngeal  ring.  This  tissue  is  divided  into  three  main 
masses — that  lying  in  the  base  of  the  tongue  forming  the  lingual 
tonsil,  that  associated  with  the  palate  and  lying  between  the  pillars 
of  the  pharynx  and  forming  the  palatine  tonsil,  and  that  situated 
in  the  pharynx  or  pharyngeal  tonsil. 

The  Lingual  Tonsils  — These  are  situated  in  the  base  of  the 
tongue  between  the  circumvallate  papillae  and  the  epiglottis.  They 
are  rounded  masses  of  adenoid  tissue  composed  of  solitary  follicles 
lying  mostly  in  the  tunica  propria,  and  causing  projections  of  the 
surface  that  are  easily  seen.  In  the  center  of  each  mass  is  a  deep 
depression  forming  a  blind  pouch,  known  as  the  crypt  (Fig.  249). 
This  is  lined  with  stratified  squamous  epithelium  like  that  of  the 
adjoining  mucous  membrane  except  that  at  various  places  the 
lymphocytes  have  pushed  their  way  through  the  epithelial  cells, 
and  escape  on  the  surface. 

The  Palatine  Tonsils. — These  lie  at  the  base  of  the  tongue  between 
the  anterior  and  the  posterior  pillars  of  the  pharynx.  They  are 
much  larger  than  the  lingual  tonsils  and  are  composed  of  from  ten 
to  twenty  follicles  and  a  number  of  crypts.  The  epithelium  cover- 
ing them  is  pierced  in  many  places  by  encroachments  of  the  adenoid 
tissue.  The  crypts  always  contain  many  lymphocytes  (Fig.  250). 
These  are  what  are  ordinarily  called  the  tonsils,  the  infection  of 
which  produces  tonsillitis. 

The  Pharyngeal  Tonsils. — These  lie  on  the  posterior  wall  of  the 
nasal  pharynx  above  the  level  of  the  palate.  Their  structure  is 
similar  to  that  of  the  palatine  tonsil.  The  crypts  are  five  to  six 
in  number  and  are  often  clothed  with  ciliated  epithelium.  Into 
them  open  the  ducts  of  mixed  glands  which  form  a  distinct  layer 
under  the  follicle.  Here  also  there  is  a  migration  of  lymphocytes 
through  the  epithelium.  It  is  the  hypertrophy  of  these  which  form 
the  adenoids  so  often  found  in  children. 


CHAPTER  XXIV. 

BIOLOGICAL  CONSIDERATIONS  FUNDAMENTAL  TO 
EMBRYOLOGY. 

History. — Before  beginning  the  study  of  embryology  some  topics 
in  general  histology  must  be  reviewed,  and  some  general  biologic 
ideas  considered.  No  real  conception  of  the  complicated  process 
of  individual  development  can  be  obtained  without  laying  a  founda- 
tion in  the  study  of  the  cell  as  the  units  of  life  and  the  mechanism 
through  which  the  phenomena  of  life  are  manifested. 

In  embryology  it  is  found  that  the  individual  in  his  physical 
development  passes  through  stages  which  correspond  to  the  develop- 
ment of  the  race  or  species  to  which  he  belongs,  and  a  like  compari- 
son might  be  drawn  in  mental  development  and  the  acquirement 
of  knowledge.  This  is  specially  true  of  the  subject  of  embryology. 

Apparently  the  first  ideas  to  occupy  the  speculative  thought 
of  man  when  he  became  conscious  of  himself  as  an  independent 
being  were  the  questions  of  his  origin  and  the  relation  to  his  environ- 
ment and  destiny.  These  have  become  the  basis  for  the  development 
of  all  religious  thought. 

Up  to  the  beginning  of  the  nineteenth  century  all  considerations 
of  these  subjects  were  purely  speculative.  The  old  question  of 
"What  is  life?"  received  endless  discussion.  In  the  nineteenth 
century  this  question  has  been  dropped  into  the  background,  and 
the  question,  "What  is  the  mechanism  of  life?"  has  been  substituted 
for  it.  The  consideration  of  the  latter  question  has  resulted  not 
only  in  the  marvellous  advancement  of  medical  knowledge  and 
surgical  skill,  but  in  the  great  development  of  deeper  fundamental 
thoughts.  It  must  not  be  forgotten,  however,  that  the  develop- 
ment of  knowledge  resulting  from  the  consideration  of  the  latter 
question  has  not  and  does  not  promise  to  answer  the  old  question, 
"What  is  life?"  any  more  than  the  laws  of  electricity  and  their 
application  to  its  use  answer  the  question,  "What  is  electricity?" 

The  discovery  of  the  cell  hypothesis  and  the  propounding  of 
the  theory  of  organic  evolution  have  been  the  greatest  factors 
in  the  unification  of  knowledge  and  the  stimulation  of  thought  in 
these  fields.  It  is  interesting  to  notice  that  these  two  theories, 
closely  related  as  they  have  become,  had  entirely  independent 
(298) 


RELATION  OF  NUCLEUS  TO  PROTOPLASM  299 

origins  and  were  long  followed  out  without  any  immediate  con- 
nection. The  theory  of  evolution  was  based  upon  consideration 
of  the  forms  of  living  things,  their  distribution  and  adaptation  to 
environment. 

The  Cell  Theory. — The  cell  theory  had  its  origin  in  the  study  of 
minute  forms.  Its  beginnings  were  made  possible  by  the  develop- 
ment of  the  compound  microscope,  which  revealed  their  structure 
and  showed  them  to  be  small  bodies  made  up  of  apparently  a  struct- 
ureless, granular  material  which  was  called  protoplasm,  or  the 
ultimate  substance  of  life.  This  material,  as  its  name  indicates 
was  originally  supposed  to  be  simple  in  structure  and  composition 
and  to  be  the  life  substance.  Huxley's  characterization  of  it  as 
the  "physical  basis  of  life"  was  the  beginning  of  the  study  which 
has  revealed  it  to  be  very  far  from  a  simple  substance,  but  rather 
extremely  complex  both  in  structural  arrangement  and  chemical 
composition.  In  more  recent  biology,  therefore,  the  word  proto- 
plasm is  being  dropped  and  the  word  cytoplasm  or  cell  substance 
substituted  for  it- 

The  early  history  of  the  cell  theory  was  obstructed  in  its  develop- 
ment by  the  remains  of  the  old  Greek  idea  that  living  things  could 
originate  from  non-living  matter,  that  the  swamp  breeds  disease, 
and  the  decomposing  body  of  an  animal,  maggots.  It  required 
fifty  years  of  work  on  the  cell  theory  for  Virchow,  in  1850,  to  pro- 
pound his  thesis  that  all  living  cells  are  derived  from  a  preexisting 
cell,  and  so  establish  the  continuity  of  life,  which  has  flowed  on 
from  the  beginning  in  an  uninterrupted  stream,  each  individual 
being  only  a  period. 

When  Schwann  and  Schleiden  showed  that  the  bodies  of  both 
plants  and  animals,  instead  of  being  made  up  of  homogeneous  tissue, 
were  composed  of  millions  of  structural  elements  which  they  called 
cells,  the  consideration  of  both  plants  and  animals  were  for  the 
first  time  put  upon  a  common  basis.  Naturally  enough  the  first 
thing  to  attract  attention  was  the  study  of  the  form  and  arrange- 
mentof  these  structural  elements  in  the  tissuesof  animals  andplants. 

In  following  out  this  study  it  became  more  and  more  evident 
that,  while  infinitely  varied  in  the  detail  of  their  form  and  structure, 
all  cells  had  a  common  plan  of  organization  and  possessed  struct- 
ural characteristics  common  to  all,  at  least  in  some  stages  of  their 
history. 

Relation  of  the  Nucleus  to  the  Protoplasm. — The  first  point  to  be 
discovered  in  the  internal  organization  of  the  cell  was  the  nucleus, 


300        BIOLOGICAL  CONSIDERATIONS  OF  EMBRYOLOGY 

the  meaning  of  which  and  its  relation  to  the  cytoplasm  at  once 
attracted  attention.  As  the  result  of  a  vast  amount  of  work,  it 
was  gradually  established  that  the  nucleus  "exerts  a  controlling 
and  directing  influence  over  the  activity  of  the  cytoplasm;"  that 
a  cell  deprived  of  its  nucleus  would  continue  to  live  for  a  longer  or 
shorter  time,  but  that  it  would  not  grow  and  would  not  reproduce 
another  cell;  that  the  phenomena  of  life  manifested  by  destructive 
metabolism  would  continue  until  the  identity  of  the  cytoplasm 
was  destroyed,  but  there  would  be  no  constructive  metabolism. 
The  work  of  the  cytoplasm  is  therefore  dependent  upon  the 
character  of  the  nuclear  material. 

Cell  Division. — As  first  observed,  cell  division  was  supposed  to 
be  an  irregular  cutting  of  the  cytoplasm  and  the  nucleus  in  two, 
forming  two  individual  cells.  The  cytoplasm  by  its  constructive 
changes  does  not  continue  to  increase  indefinitely,  but  as  soon  as 
a  certain  size  is  reached  it  divides,  a  portion  of  the  nucleus  going 
to  each  of  the  parts,  which  immediately  begin  to  increase  in  size. 
It  was  soon  found  that  cell  division  was  not  always  so  simple,  and 
that  in  some  cases  changes  in  the  nucleus  preceded  the  division  of 
the  cytoplasm.  Two  forms  of  cell  division  are  therefore  described, 
the  simple  or  direct,  and  indirect  or  karyokinetic  cell  division. 
The  simple  is  now  known  to  be  comparatively  rare. 

Indirect  Cell  Division. — Indirect  cell  division  must  be  considered 
as  a  means  by  which  the  chromatic  material  of  the  nucleus  is  equally 
and  systematically  distributed  to  the  resulting  cells.  The  nucleus, 
in  cell  division,  contains  a  beautiful  structural  mechanism,  by  which 
the  material  which  is  to  control  the  development  of  the  resulting 
cells  and  their  activity  is  definitely  distributed  to  them.  In  this 
process  there  is  no  irregularity  in  the  kind  or  amount  of  material 
given  to  the  two  cells. 

In  this  process  the  chromatin  of  the  original  nucleus  is  divided 
into  a  definite  number  of  pieces  which  are  split  in  two,  and  half 
of  each  sent  to  each  new  nucleus,  where  they  form  its  chromatin 
network. 

The  Vehicle  of  Transmission. — It  was  discovered  that  the  number 
of  chromosomes  was  constant  in  every  cell  division  for  all  the  cells 
of  all  the  tissues  of  the  given  species,  and  was,  therefore,  a  charac- 
teristic of  the  species;  and  that  in  all  the  cells  of  the  body  it  was 
always  an  even  number,  and  that  in  the  germ  cells  of  the  species 
the  number  of  chromosomes  was  exactly  half  that  in  the  cells  of 
the  body.  This  led  to  the  immediate  recognition  of  the  chromatic 


CHEMICAL  IDEAS  301 

material  as  the  vehicle  of  transmission.  When  in  the  study  of  fer- 
tilization it  was  found  that  fertilization  consists  in  the  union  of 
two  cells,  each  contributing  both  cytoplasm  and  nucleus,  and  that 
the  amount  of  chromatic  material  was  equal  from  each,  and  exactly 
half  that  found  in  the  cells  of  the  parent  body,  the  equality  of  the 
sexes  in  transmission  was  firmly  established  upon  a  cytologic  basis. 
It  is  interesting  to  note  that  this  equality  had  previously  been 
claimed  by  the  disciples  of  the  evolutionary  theory,  and  it  was  in 
this  field  that  the  evolutionary  theory  and  the  cell  theory  first 
met  on  common  grounds  (about  1875). 

All  the  advancement  in  modern  thought  concerning  heredity 
and  transmission  has  resulted  from  these  discoveries.  The  practical 
results  are  perhaps  still  more  important  in  the  artificial  breeding 
of  plant's  and  animals,  adapting  them  to  their  environment.  The 
work  of  such  men  as  Burbank  may  be  said  to  be  the  application 
of  the  knowledge  of  the  mechanism  of  cell  division  and  inheritance 
to  horticulture  and  agriculture. 

Chemical  Ideas. — At  the  present  time  the  structural  mechanism 
of  life,  while  inviting  many  fields  for  research,  may  be  said  to  have 
nearly  reached  the  limit  of  possibilities  of  observation,  and  at  the 
present  time  the  chemical  phase  is  attracting  the  greatest  attention. 
Such  questions  as,  "How  does  the  nucleus  influence  the  activity 
of  the  cytoplasm?"  are  being  eagerly  investigated.  Cytoplasm 
while  enormously  complex  in  chemical  composition,  must,  never- 
theless, always  be  thought  of  as  performing  its  vital  functions  by 
chemical  activity.  It  is  constantly  building  simpler  molecules  into 
its  own,  and  so  increasing  in  amount.  For  this  its  surface  must  be 
bathed  in  materials  with  which  it  can  react.  It  is  evident  that  if 
the  mass  increased  indefinitely  the  volume  would  increase  much 
more  rapidly  than  the  surface,  and  this  puts  a  limit  upon  the 
growth. 

The  constructive  metabolism  of  the  cytoplasm  is  dependent 
upon  the  presence  of  the  chromatin  in  the  nucleus.  In  the  process 
of  metabolism,  therefore,  there  must  be  interaction  between  the 
chemical  substances  of  the  chromatin,  cytoplasm,  and  food  material. 
The  development  of  physiologic  chemistry  is  rapidly  affecting  the 
ideas  of  the  cause  and  treatment  of  disease,  and  especially  the 
production  of  immunity  and  susceptibility. 

If  the  dental  profession  is  to  keep  pace  with  the  development 
in  these  fields  and  apply  the  results  of  investigation  to  the  treat- 
ment of  diseases  of  the  mouth,  the  study  of  the  fundamental  sciences 
must  be  more  thorough. 


CHAPTER  XXV. 
EARLY  STAGES  OF  EMBRYOLOGY. 

SINCE  fertilization  consists  essentially  in  the  union  of  the  chroma- 
tin  from  two  cells,  and  as  the  result  of  the  union  restores  the  normal 
amount  of  chromatin  for  the  cells  of  that  species,  it  is  evident  that 


FIG.  251. — Diagram  illustrating  the  reduction  of  the  chromosomes  during  the 
maturation  of  the  ovum:  o,  ovum;  oc1,  odcyte  of  the  first  generation;  oc2,  oocyte  of 
the  second  generation;  p,  p,  polar  bodies.  (McMurrich.) 

in  some  way  the  germ  cells  must  be  prepared  for  fertilization  by  the 
loss  of  half  their  chromatin.     This  process  was  first  observed  in 
the  case  of  the  ovum. 
(302) 


MATURATION 


303 


Maturation. — In  observing  fertilization  of  eggs  of  the  starfish 
and  various  threadworms,  it  was  noticed  that  before  fertilization 
occurred  the  nucleus  of  the  ovum  divided  with  karyokinetic  figures, 
forming  three  small  bodies  known  as  polar  bodies.  This  process 
is  diagrammed  in  Fig.  251.  In  reality,  the  ovum  first  divides, 
forming  one  polar  body;  the  polar  body  and  the  ovum  both  then 
divide  again,  so  that  the  result  of  the  two  series  of  division  is  the 


FIG.  252. — Diagram  illustrating  the  reduction  of  the  chromosomes  during  sperma- 
togenesis:  sc1,  spermatocyte  of  the  first  order;  sc2,  spermatocyte  of  the  second  order; 
sp,  spermatid.  (McMurrich.) 

formation  of  four  cells,  one  of  which  is  functional,  three  disappear- 
ing. This  process  is  practically  universal  in  the  formation  of  ova 
of  both  plants  and  animals.  The  cells  in  the  ovary  which  form  the 
ova  are  called  oogonia.  The  cells  formed  from  these  are  the  primary 
oocyte.  The  division  of  this  cell  produces  two  secondary  oocytes, 
of  which  one  disappears  later.  The  division  of  the  secondary  oocyte 
results  in  the  ovum  and  three  polar  bodies.  The  number  of  chromo- 
somes in  the  primary  oocyte  is  half  the  number  characteristic  of 


304 


EARLY  STAGES  OF  EMBRYOLOGY 


the  somatic  cells,  but  they  are  made  up  of  four  pieces.  In  the 
secondary  oocytes  they  are  the  same  number  but  double.  In  the 
ovum  and  polar  bodies  they  are  the  same  in  number  and  single. 

Spermatogenesis.- — Exactly  the  same  series  of  changes  occur 
in  the  formation  of  the  spermatozoa.  They  are  illustrated  in  Fig. 
252.  On  the  outer  wall  of  the  seminiferous  tubules  are  two  forms 
of  cells,  the  spermatogonia  and  the  cells  of  Sertoli  (Fig.  253).  The 
cell  of  Sertoli  increases  in  size  and  spreads  out  against  the  basement 
membrane,  pushing  the  spermatogonia  away  from  it.  They  now 
divide,  forming  two  cells,  one  of  which  returns  to  the  basement 
membrane  and  remains  as  the  spermatogonia,  the  other  becomes  a 
primary  spermatocyte.  The  primary  spermatocytes  divide,  form- 
ing a  secondary;  the  secondary  divide,  forming  spermatids,  which 


FIG.  253. — Diagram  showing  stages  of  spermatogenesis  as  seen  in  different  sections 
of  a  seminiferous  tubule  of  a  rat:  s,  sertoli  cell;  sc1,  spermatocyte  of  the  first  order; 
sc2,  spermatocyte  of  the  second  order;  sg,  spermatogone ;  sp,  spermatid;  sz,  sperma- 
tozoon. (Von  Lenhossek's  diagram  from  McMurrich.) 

develop  directly  into  spermatozoa.  By  comparing  the  diagrams 
they  will  be  seen  to  correspond  exactly  with  the  formation  of  the 
ova,  except  that  all  of  the  cells  are  small  and  motile.  The  nuclear 
changes  also  correspond  to  those  of  the  ova,  the  primary  sperma- 
tocyte having  half  the  number  of  tetrad  chromosomes,  the  second- 
ary half  the  number  of  diad,  and  the  spermatids  half  the  number 
of  monad  chromosomes. 

Fertilization. — Fertilization  is  essentially  the  same  in  the  sexual 
reproduction  of  all  plants  and  animals.  It  may  be  easily  observed 
in  the  transparent  cells  of  such  animals  as  the  starfish  and  the 
threadworm,  The  spermatozoon  enters  the  cytoplasm  of  the  ova, 


FERTILIZATION 


305 


-pb 


FIG.  254. — Fertilization  of  the  egg  of  Ascaris  megalocephala,  var.  bivalens.  (Boveri.) 
A,  the  spermatozoon  has  entered  the  egg;  its  nucleus  is  shown  at  J  ;  beside  it  lies  the 
granular  mass  of  "archoplasm"  (attraction  sphere);  above  are  the  closing  phases  in 
the  formation  of  the  second  polar  body  (two  chromosomes  in  each  nucleus).  B,  germ 
nuclei  (S,  tf)  in  the  reticular  stage;  the  attraction  sphere  (a)  contains  the  dividing 
centrosome.  C,  chromosomes  forming  in  the  germ  nuclei;  the  centrosome  divided. 

D,  each  germ  nucleus  resolved  into  chromosomes;    attraction  sphere   (a)   double. 

E,  mitotic  figure  forming  for  the  first  cleavage;    the  chromosomes  (c)  already  split. 

F,  first  cleavage  in  progress,   showing  divergence  of  the  daughter    chromosomes 
toward  the  spindle  poles  (only  three  chromosomes  shown).     (Wilson.) 

20 


306 


EARLY  STAGES  OF  EMBRYOLOGY 


where  it  immediately  loses  its  characteristic  form  and  develops 
into  a  typical  nucleus  (Fig.  254).  The  ovum  now  has  two  nuclei, 
one  of  which  is  called  the  male  pronucleus,  the  other  the  female 
pronucleus.  These  both  form  chromosomes,  the  number  from  each 
being  half  the  number  typical  of  the  species.  These  are  arranged 
as  usual  between  the  centrosomes.  They  divide  longitudinally, 
each  forming  two,  one  of  which  passes  to  either  centrosome,  where 
a  new  nucleus  is  formed,  and  in  the  meantime  the  cytoplasm  has 
divided  so  that  two  cells  are  formed.  The  nuclear  material  of  these 
two  cells  has  therefore  been  equally  derived  from  the  two  parents, 
and  it  is  to  control  all  of  the  future  development  of  the  individual. 


FIG.  255. — Holoblastic  segmentation.    Segmentation  of  frog  diagrammatically 

represented. 


SEGMENTATION. 

Holoblastic  Segmentation. — An  idea  of  the  development  of  the 
embryo  can  perhaps  best  be  obtained  by  following  the  development 
of  the  frog.  The  frog's  eggs  are  large  and  easily  observed,  and  they 
contain  only  a  small  amount  of  yolk  or  food  material,  which  does 
not  obstruct  the  observation.  The  spherical  ovum  first  divides 
into  hemispheres;  these  two  cells  are  divided  into  four  in  a  plane 
at  right  angles,  and  the  four  are  divided  into  eight  by  a  plane  at 
right  angles  to  the  previous  plane.  This  is  best  understood  by 
examining  the  illustration  (Fig.  255). 

The  lines  of  cell  division  proceed  in  a  regular  way,  the  planes 
passing  in  such  direction  as  to  multiply  the  number  of  cells  by  two 
in  each  set  of  divisions.  Very  soon  the  cells  around  the  black  pole 
show  a  tendency  to  divide  more  rapidly  than  those  at  the  white 
pole.  At  this  stage  the  individual  is  made  up  of  a  hollow  sphere 
of  cells  with  a  space  at  the  center,  the  cells  at  the  upper  surface 


HOLOBLASTIC  SEGMENTATION 


307 


being  small  and  rapidly  dividing,  those  at  the  lower  surface  large 
and  slowly  dividing  (Fig.  256) .  As  this  continues  the  sphere  becomes 
flattened  on  the  bottom,  and  finally  the  lower  surface  is  turned 
inward  until  the  sphere  is  converted  into  a  hollow  bag  or  sac  made 
up  of  two  layers  of  cells,  the  outer  of  which  are  small,  the  inner 


FIG.  256. — Four  stages  in  the  development  of  amphioxus,  illustrating  the  forma- 
tion of  the  gastrula.  I,  the  blastula,  a  hollow  sphere  of  cells;  those  at  the  lower  pole 
larger  than  those  at  the  upper  and  filled  with  yolk  granules.  II,  invagination  of  the 
lower  pole,  because  of  more  rapid  growth  of  cells  at  the  upper  pole.  Ill,  the  gas- 
trula, complete  invagination;  the  creature  is  now  a  two-layered  bag.  A  space 
should  be  shown  between  the  layers:  bl,  the  mouth  of  the  bag,  or  blastopore;  hy, 
inner  layer  of  cells — hypoblast;  ep,  outer  layer  of  cells — epiblast.  IV,  the  gastrula 
will  now  elongate;  the  cavity  becomes  the  alimentary  canal;  the  blastopore  the 
orifice  at  one  end. 

large,  the  two  joining  around  the  mouth  of  the  sac.  This  hollow 
bag  stage  is  known  as  the  gastrula.  The  cavity  of  the  sac  is  really 
a  part  of  the  outside  world  around  which  the  cells  have  grown,  and 
will  form  the  cavity  of  the  alimentary  canal.  The  opening  of  the 
sac  is  known  as  the  blastopore,  and  will  form  the  anterior  opening 


308  EARLY  STAGES  OF  EMBRYOLOGY 

into  the  alimentary  tract  from  the  mouth  cavity.  At  this  stage 
the  individual  is  made  up  of  two  kinds  of  cells,  and  is  to  be  compared 
in  structure  with  the  celenterates  or  such  animals  as  the  fresh- 
water hydra  and  the  coral  polyp. 

Formation  of  the  Germ  Layers. — The  cells  which  form  the  outer 
layer  of  the  gastrula  are  called  the  epiblast,  the  cells  which  line  it 
the  hypoblast  or  entoblast.  Where  these  two  layers  join  around 
the  opening  of  the  blastopore  a  ring  of  cells  is  formed  which  differs 
from  both  in  form  and  arrangement,  and  will  form  the  mesoblast. 
In  the  process  of  cell  division  from  the  ovum,  therefore,  three 
kinds  of  cells  have  resulted  which  represent  the  first  stage  of 
specialization. 

Epiblast. — From  the  cells  of  the  epiblast  will  be  formed:  (1) 
The  epithelium  of  the  surface  of  the  body  and  all  glands  that  con- 
nect with  it,  the  hair,  the  nails,  and  the  enamel  of  the  teeth;  (2) 
the  epithelium  lining  the  mouth  and  the  nose  cavities  and  the 
lower  part  of  the  rectum;  (3)  the  nervous  system  and  all  of  the 
organs  of  special  sense. 

Hypoblast. — From  the  hypoblast  will  be  formed:  (1)  The 
epithelium  lining  the  alimentary  canal  and  the  glands  that  open 
from  it;  (2)  the  epithelium  lining  the  larynx,  trachea,  and  the 
lungs;  (3)  the  epithelium  of  the  bladder  and  ureter. 

Mesoblast. — From  the  mesoblast  will  be  formed :  (1)  The  various 
connective  tissues,  including  bone,  dentin,  and  cementum;  (2) 
the  muscles,  both  striated  and  unstriated;  (3)  the  circulatory 
system,  including  the  blood  itself  and  the  lymphatics;  (4)  the 
lining  membrane  of  the  serous  cavities  of  the  body;  (5)  the  kidney; 
(6)  the  internal  organs  of  reproduction. 

Looking  at  these  germ  layers  in  another  way,  it  may  be  said 
that  through  the  mechanism  of  cell  division  all  of  the  chromatin 
which  is  to  control  nerve  cytoplasm  has  been  distributed  to  the 
epiblast;  all  that  which  is  to  contribute  the  muscular  activity  to 
the  mesoblast,  and  so  on. 

Meroblastic  Segmentation. — If  the  development  of  the  chick  is 
compared  to  that  of  the  frog  they  at  first  seem  to  be  very  different. 
The  ova  of  birds  and  reptiles  are  provided  with  a  vast  amount  of 
food  material  or  yolk,  which  is  provided  by  the  parent  for  the 
nourishment  of  the  embryo.  It  has  been  seen  that  the  frog's  egg 
contains  a  certain  amount  of  yolk,  and  that  the  presence  of  yolk 
granules  retarded  the  cell  division.  In  the  case  of  the  birds  and 
reptiles  the  yolk  granules  have  increased  until  the  active  cytoplasm 


ME  ROB  LAST  1C  SEGMENTATION 


309 


is  left  as  a  small  disk  floating  on  top  of  a  sphere  of  yolk  enclosed 
in  the  yolk  membrane.  The  white  spot  seen  floating  on  the  top 
of  the  yolk  of  a  hen's  egg  is  called  the  germinal  spot.  Before  fertili- 


FIG.  257. — Meroblastic  segmentation. 

zation  this  is  a  mass  of  protoplasm  with  a  nucleus  in  the  center 
When  segmentation  begins  it  divides  first  into  right  and  left  halves 


FIG.  258. — First  five  stages  of  segmentation  (rabbit's  ovum),  a,  b,  c,  d,  and  e. 
In  a,  b,  and  c  the  epiblast  cells  are  larger  than  the  hypoblastic  ones.  In  e  the 
epiblast  cells  have  become  smaller  and  more  numerous  than  the  hypoblasts  and 
the  epiblastic  spheres  are  beginning  to  surround  and  close  in  the  hypoblast  cells: 
z.p.,  zona  pellucida;  p.gl,  polar  globules;  u,  first  epiblast  cell;  I,  first  hypoblast  cell. 


then  divides  again  by  a  line  at  right  angles  to  the  first  one,  then 
the  four  cells  are  converted  into  eight  cells,  as  if  by  a  circle,  and 
the  process  continues  in  this  way  (Fig.  257).  It  is  best  understood 


310 


EARLY  STAGES  OF  EMBRYOLOGY 


from  the  diagram.    This  type  of  segmentation  is  known  as  mero- 
blastic,  while  that  of  the  frog  is  holoblastic. 

Mammalian  Segmentation. — The  mammalian  ova  contain  very 
little  yolk,  as  the  nourishment  of  the  embryo  is  provided  for  in 
an  entirely  different  way.  The  segmentation  is  holoblastic  (Fig. 


FIG.  259. — Sections  of  the  ovum  of  a  rabbit  during  the  later  stages  of  segmentation, 
showing  the  formation  of  the  blastodermic  vesicle:  a,  gastrula  stages;  ent,  hypo- 
blast  enclosed  by  ep,  epiblast;  b,  fluid  is  beginning  to  collect  and  separate  the  epi- 
blast  and  hypoblast;  c,  the  fluid  has  greatly  increased  in  amount,  the  hypoblastic 
cells  adhering  to  the  upper  surface;  d,  the  blastodermic  vesicle;  ect,  the  outer  layer, 
epiblast;  ent,  hypoblast,  the  inner  layer  adhering  to  the  inner  surface  of  the  epiblast 
at  the  upper  surface,  forming  the  opaque  area. 

258),  but  shows  marked  differences  from  that  of  the  frog,  and 
characteristics  similar  to  those  of  the  birds  and  reptiles,  and  this 
has  been  an  added  link  to  the  evidence  of  the  evolutionists,  that 
the  mammalia  have  been  derived  in  evolution  from  the  reptiles. 

After  the  first  few  divisions  the  cells  of  the  upper  pole  divide 
much  more  rapidly  than  those  of  the  lower,  and  grow  down  over 


MAMMALIAN  SEGMENTATION 


311 


FIG.  260. — A  series  of  sections  through  the  neurenteric  and  notochordal  canal  of  a 
mole  embryo:  p.gr.,  the  primitive  groove;  ep.,  epiblast;  me.,  mesoblast;  hy.,  hypoblast: 
m.gr.,  medullary  groove.  (Heap.) 


312  EARLY  STAGES  OF  EMBRYOLOGY 

the  others,  enclosing  them.  When  the  large  cells  have  been  entirely 
covered  in  by  the  small  ones,  the  small  ones  continue  to  multiply 
more  rapidly  and  fluid  collects  inside  the  sphere,  leaving  the  large 
cells  adhering  to  the  inner  surface  of  the  small  cell  layer  at  one  pole 
of  the  sphere  (Fig.  259).  At  the  upper  pole  where  the  sphere  is 
made  up  of  two  layers  of  cells  there  is  an  opaque  spot,  or  the  "area 
pellucida,"  from  only  part  of  which  the  embryo  is  developed,  the 
rest  forming  organs  to  provide  it  with  nourishment  during  the 
embryonal  condition. 

Starting  from  the  center  of  the  opaque  area  on  the  upper  surface 
of  the  sphere  or  blastula,  there  appears  a  streak  known  as  the 
primitive  streak,  caused  by  the  appearance  of  a  rod  of  cells  lying 
between  the  two  layers,  and  from  the  side  of  this  rod  or  notochord 
a  third  kind  of  cell,  different  from  either  the  large  or  small  cell 
layer,  is  formed.  These  three  kinds  of  cells  make  up  the  three 
layers  of  the  blastoderm  and  represent  the  first  step  in  differentia- 
tion; or,  to  state  it  in  a  different  way,  all  of  the  chromatin  which 
(Fig.  260)  directs  nerve  cell  activity  has  been  sent  to  the  outer 
small  cell  layer,  or  epiblast,  all  of  the  chromatin  which  directs 


LEGEND  FOR  PLATE  XX 

FIGS.  1  to  5. — Diagrammatic  representations  of  longitudinal  and  cross-sections  of 
hen's  egg  in  various  stages  of  incubation.  They  illustrate  how  the  embryo  is  devel- 
oped out  of  the  area  pellucida,  and  the  yolk  sac,  the  serosa,  and  the  allantois  out 
of  the  extra-embryonal  area  of  the  germ  layers.  The  embryo  is  represented  much 
too  large  in  relation  to  the  yolk  sac.  The  yolk  is  represented  in  yellow  and  the  ento- 
derm  in  green,  ectoderm  in  blue,  mesoderm  in  red,  and  the  black  dotted  lines  indi- 
cate the  limit  to  which  the  inner  and  outer  germ  layers  have  extended  over  the  yolk. 
The  red  dots  mark  the  limit  of  the  mesoderm:  ak,  outer  germ  layer  (blue);  mw, 
medullary  ridges  or  folds;  A7",  neural  tube;  am,  amniotic  fold;  vof,  hof,  saf,  anterior, 
posterior,  and  lateral  amniotic  folds;  A,  amnion,  ah,  amniotic  cavity;  S,  serous 
membrane;  hu,  dermal  umbilicus;  sf,  lateral  folds;  kf  1,  kf  2,  head  fold;  afb,  ifb, 
outer  and  inner  limb  fold;  ik,  inner  germ  layer  (green) ;  ir,  its  margin  of  overgrowth; 
dr,  intestinal  groove;  dg,  vitelline  duct;  al,  allantois;  ds,  interstitial  sac;  du,  intes- 
tinal umbilicus;  mk,  middle  germ  layer  (red);  mk,  parietal  layer  of  mesoderm;  mk, 
visceral  layer  of  mesoderm;  si,  lateral  limits  of  the  same;  dm,  vm,  dorsal  and  ventral 
mesenteries;  th',  body  cavity;  th1,  </i2,  embryonic  extra-embryonic  parts  of  the  same. 

FIG.     1. — Cross-section  through  hen's  egg  on  second  day  of  incubation. 

FIG.    2. — Cross-section  through  hen's  egg  on  third  day  of  incubation. 

FIG.    3. — Longitudinal  section  through  hen's  egg  on  third  day  of  incubation. 

FIG.  4. — Longitudinal  section  through  hen's  egg  beginning  of  fourth  day  of 
incubation. 

FIG.    5. — Longitudinal  section  through  hen's  egg  on  seventh  day  of  incubation. 

FIG.    6. — Cross-section  through  embryo,  first  day. 

FIG.    7. — Diagrammatic  longitudinal  section  through  a  selachian  embryo. 

FIG.     8  (Kollikie).' — Half  of  a  cross-section  through  embryo  chick  (two  days.) 

FIG.    9  (Kollikie). — Cross-section  through  embryo  chick,  beginning  of  third  day. 

FIG.  10. — Cross-section  of  chick  (five  days)  in  the  region  of  the  umbilicus. 

FIG.  11. — Diagrammatic  longitudinal  section  of  the  embryo  chick. 


PLATE    XX 


0.1  'f 


BRANCHIAL  ARCHES  313 

muscle  cell  activity,  etc.,  has  been  sent  to  the  new  cells  of  the 
third  layer,  or  mesoblast,  while  the  large  cells  of  the  inner  layer  or 
hypoblast  contain  chromatin  to  direct  most  of  the  secretory  activ- 
ities and  the  formation  of  the  epithelium  of  the  alimentary  canal. 


NERVOUS    SYSTEM. 

Formation  of  Neural  Canal. — The  epidermal  cells  of  either  side 
of  the  primitive  streak  grow  rapidly,  forming  two  ridges  with  a 
groove  between  them,  which  grows  deeper  and  deeper  until  the 
ridges  bend  over  and  join,  enclosing  a  tube  which  is  to  be  the 
canal  of  the  spinal  cord  (Fig.  261).  The  anterior  end  of  this  tube 
enlarges  into  three  bulbs  which  correspond  to  the  ventricles  of 
the  brain,  and  as  they  increase  in  size  they  fold  over  ventrally  or 
toward  the  center  of  the  sphere  until  the  first  and  second  are  at 
right  angles  to  the  original  tubular  part. 

As  the  outer  layer  forms  the  tube  of  the  central  nervous  system, 
the  inner  layer  folds  off  a  blind  pouch  from  the  general  cavity  of 
the  sphere  which  is  to  form  the  anterior  part  of  the  alimentary 
canal  (Plate  XX).  By  this  time  development  is  complicated  by 
the  formation  of  the  embryonal  membranes,  the  amnion  and  allan- 
tois,  but  we  may  omit  these  entirely  for  our  purposes. 

The  diagram  from  Quain's  Anatomy  (Figs.  262  and  263)  illus- 
trates the  condition  just  described,  showing  the  embryo  in  longi- 
tudinal section,  the  bending  over  of  the  anterior  end  of  the  neural 
canal  to  form  the  mid-  and  forebrain  and  the  foregut,  or  esophagus, 
a  blind  pouch  ending  anteriorly  under  the  midbrain  and  posteriorly 
opening  into  the  cavity  of  the  sphere  now  called  the  yolk  sac.  This 
pouch  is  lined  by  hypoblast  and  covered  by  mesoblast  and  epiblast. 
The  heart  has  already  begun  its  development  in  the  mesoblast  on 
the  ventral  side  of  the  foregut. 

Branchial  Arches. — There  nowr  appear  what  are  called  the  gill 
slits,  openings  from  the  foregut  through  its  walls  to  the  surface  of 
the  embryo,  which  are  separated  by  thickenings  of  the  wall  forming 
arches  around  the  gut  known  as  the  visceral  or  branchial  arches, 
at  the  center  of  each  of  which  is  found  a  bloodvessel.  These  struct- 
ures are  to  be  compared  to  the  gills  of  a  fish,  which  are  slits  through 
the  wall  of  the  esophagus  to  the  outside,  so  that  water  taken  into 
the  mouth  may  pass  out  through  the  slits.  At  this  time,  too,  the 
arrangement  of  the  bloodvessels  exactly  resembles  that  of  a  fish, 


314 


EARLY  STAGES  OF  EMBRYOLOGY 


and  the  individual  may  be  said  to  be  in  the  fish  stage  of  develop- 
ment. 


hy. 


FIG.  261. — Stages  in  the  conversion  of  the  medullary  groove  into  the  neural  canal. 
From  tail  end  of  embryo  of  the  cat:  m.g.,  medullary  groove;  n.c.,  neural  canal; 
ch.,  notochord;  ep.,  epiblast;  hy.,  hypoblast;  me.,  mesoblast;  cce.,  celom;  am.,  amnion. 
(After  Quain.) 


Stomodeum. — Plate  XXI,  from  Quain's  Anatomy,  and  Fig.  264, 
from  Hertwig's  Text-book  of  Embryology,  shows  the  embryo  at 
this  stage  and  the  arrangement  of  the  bloodvessels.  As  the  fore- 


PLATE  XXI 


First  aortic  arch 


Auditory  vesicle 


Primitive  jugular  vein 
Fourth  aortic  arch 

Sixth  aortic  arch 
Dorsal  aorta 


Cardinal  vein 


Digestive  tube 


Hind-gut 


Olfactory  pit 


SI  axillary  iirocexx 
Fir>it  branchial  yroove 
Mandibular  arch 

Bulbil*  eordin 
A  triu~  ;i 
Duct  of  Cuvicr 
Ventricle 


Allantois 
Umbilical  artery 


Profile  View  of  a  Human  Embryo   Estimated  at  Twenty-one 
Days  Old.     (After  His) 


Si 


.lowing    branchial   arches   and   relation   to    bloodvessels. 


STOMODEUM 


315 


brain  grows  ventrally,  the  first  visceral  arch,  or  mandibular  arch, 
also  grows  in  the  same  direction,  and  the  space  between  the  inferior 
surface  of  the  forebrain  and  the  upper  surface  of  the  first  arch  is 
the  beginning  of  the  mouth  and  nose  cavities,  now  called  the  stomo- 
deum.  From  the  base  of  the  mandibular  arch  is  seen  also  the 
rounded  bud,  which  is  beginning  to  grow  forward  along  the  base 
of  the  forebrain  to  form  part  of  the  maxillary  arch,  and  finally  the 
upper  jaw.  At  this  time  also  the  area  which  is  to  develop  the 


,4  m  n  i 


-Allantois 
Hind-gut 


FIG.  262.- — Diagram  of  a  longitudinal  section  of  a  mammalian  embryo.     Very 
early,  showing  the  folding  off  of  the  embryo.     (After  Quain.) 


sense  of  smell  appears  on  each  side  at  the  outer  and  lower  portion 
of  the  forebrain.  The  olfactory  areas  grow  out  of  the  base  of  the 
forebrain,  at  first  being  on  the  outside  of  the  head  and  in  the  later 
development  being  enclosed,  leaving  an  opening  to  the  surface — • 
the  nostril. 

If  we  have  gained  a  correct  idea  of  the  conditions  just  described 
by  means  of  the  pictures,  it  will  be  understood  that  by  the  growing 
forward  of  the  mandibular  arch  there  is  left  an  almost  cubical 


316 


EARLY  STAGES  OF  EMBRYOLOGY 


space  between  the  lower  surface  of  the  fore-  and  midbrain  and  the 
upper  surface  of  the  mandibular  arch  (Fig.  264).    This  is  a  part 


FIG.  263. — Median  sections  through  the  head  of  embryo  rabbits  five  (A)  and  six 
(B)  millimeters  long:  A,  the  opening  from  the  foregut  has  not  yet  been  made; 
B,  the  faucial  opening  is  shown  at  /;  c,  first  brain  vesicle;  me,  midbrain  vesicle; 
mo,  medulla  oblongata;  m,  medullary  epiblast;  if,  infundibulum;  sp.e,  sphenoeth- 
noidal,  be,  sphenoidal,  and  sp.o,  spheno-occipital  parts  of  the  basal  cranii;  i,  foregut; 
ch,  notochord;  py,  buccal  pituitary  involution;  am,  amnion;  h,  heart. 

of  the  outside  world,  and  is  enclosed  to  form  the  mouth  and  nose 
cavities.    This  process  is  best  understood  if  we  think  of  the  develop- 


FIG.  264. — Embryo  showing  brachial  arches  and  stomodeum. 


ment  from  the  anterior  end  of  the  forebrain  of  a  process  which 
may  be  described  as  a  curtain  dropping  down,  making  a  central 


STOMODEUM 


317 


piece,  and  the  bud  from  the  mandibular  arch  on  each  side  growing 
forward  to  unite  \vith  it,  leaving  a  slit  between  them  and  the  man- 


Lens. 


Olfactory 
pit. 


Ma.rillary  process. 

Mandibular  arch. 

Hyo-mandibular  cleft. 


Auditory  vesicle. 
Hyoid  arch. 

Thyro-hyoid  arch. 

Sinus 

-'  prsecervicalis 


FIG.  265. — The  beginning  of  the  mandibular  arch  and  the  maxillary  buds. 


Cerebral  hemisphere. 


Fronto-nasal-. 
process. 

Stomodseum.  — ^m 


Lateral  nasal  jirocess. 

-Eye. 

~~Processus  globularis. 
Maxillary  process. 

Mandibular  arch. 
Hyo-mandibular  cleft. 


FIG.  266. — An  embryo  a  little  older  than  Fig.  265.    Viewed  from  in  front.     Showing 
development  of  maxillary  buds  and  frontonasal  process. 


dibular  arch  which  will  be  the  mouth.     In  order  to  get  a  correct 
idea  of  this  process  it  must  be  followed  somewhat  more  minutely. 


318 


EARLY  STAGES  OF  EMBRYOLOGY 


Frontonasal   Process. — As   the   frontonasal   process    develops   it 
is  made  up  of  four  rather  bulb-like  portions  (Figs.  265  and  266), 


FIG.  267. — Embryo,  a  little  older  than  Fig.  266.  A,  front  view,  frontonasal 
process,  and  maxillary  buds  about  to  unite:  1,  lateral  nasal  part  of  frontonasal 
process;  2,  maxillary  bud;  3,  mandibular  arch;  4,  hyoid  arch.  B,  the  same  embryo 
with  the  mandibular  arch  removed:  1,  horizontal  growth  of  the  maxillary  bud; 
2,  lateral  nasal  process;  3,  mesial  nasal  process;  4,  globular  processes  which  form 
the  horizontal  part  of  the  intermaxillary  bone. 

two  occupying  the  center  and  which  develop  into  the  intermaxil- 
lary bone  containing  the  incisor  teeth  and  the  center  of  the  lip; 
and  two  side  or  lateral  processes  which  grow  out  around  the  olfac- 


FIG.  268. — Head  of  an  embryo  of  about  seven  weeks.  (His.)  The  external  nasal 
processes  have  united  with  the  maxillary  and  globular  processes  to  shut  off  the 
olfactory  pit  from  the  orifice  of  the  mouth. 


tory  area  and  form  the  alse  of  the  nose  surrounding  the  nostril. 
These  do  not  unite  again  with  the  central  parts,  but  the  end  stops 


SEPARATION  OF  MOUTH  AND  NOSE  CAVITY 


319 


over  the  point  where  the  maxillary  bud  unites  with  the  central 
process  (Figs.  266  and  267).  A  failure  of  union  causes  the  deformity 
of  hare-lip,  the  opening  in  the  lip  extending  to  one,  or,  if  double, 
to  both  nostrils. 

When  the  central  part  of  the  frontonasal  process  has  united 
with  the  maxillary  bud  on  each  side  the  arch  of  the  upper  jaw  is 
complete  and  the  original  cubical  space  or  stomodeum  is  enclosed, 
leaving  only  the  slit  between  the  maxillary  and  mandibulary  arches 
which  is  to  form  the  mouth;  but  the  enclosed  space  is  in  one  cham- 
ber, there  being  no  separation  between  the  mouth  and  nose  cavities, 


Palatal  process  of  pro- 
cessus  globularis. 


Palatal  part  of  maxil- 
lary process. 

Maxillary 
process 


Processes  globularis. 
\ 


Mouth  of  olfactory 
pit,  or  nostril. 


Lens. 


.Eye. 


Mouth 
cavity. 


FIG.  269.— The  head  of  an  embryo  with  the  mandibular  arch  removed.  Looking 
up  from  the  mouth  into  the  nose  cavity.  The  union  of  the  globular  processes  forming 
the  anterior  part  of  the  palate,  and  the  horizontal  ingrowths  from  the  maxillary 
buds,  showing  the  way  in  which  they  unite  from  before  backward,  separating  the 
nose  from  the  mouth  cavity. 


The  time  of  this  development  in  the  human  embryo  may  be  placed 
at  about  the  fourth  week. 

Separation  of  Mouth  and  Nose  Cavity. — The  separation  of  the 
mouth  and  nose  cavities  occurs  by  the  development  of  horizontal 
ingrowths  from  the  three  parts  making  up  the  maxilla  and  begin- 
ning at  the  center  and  progressing  backward.  First,  a  small  trian- 
gular piece  from  the  central  part  of  globular  processes  of  the  fronto- 
nasal process,  this  uniting  with  the  horizontal  or  palatal  process 
of  the  maxillary  buds  on  each  side  until  these  reach  the  apex  of 
the  triangle,  which  will  be  the  intermaxillary  bone,  just  a  little 
way  back  in  the  palate,  and  from  here  backward  they  unite  with 
their  fellow  of  the  opposite  side.  This  is  best  seen  by  removing 


320  EARLY  STAGES  OF  EMBRYOLOGY 

the  mandibular  arch  and  viewing  the  parts  from  below  (Fig.  269, 
from  Hertwig's  Embryology). 

The  deformity  of  cleft  palate  is  then  a  later  development  than 
that  of  hare-lip,  and  either  may  occur  without  the  other,  though 
they  are  usually  found  together.  The  cleft  of  the  palate  usually 
turns  to  one  side  at  the  front,  running  out  between  the  cuspid  and 
lateral  unless  it  is  double,  when  a  detached  piece  is  found  in  the 
center  in  front,  containing  the  incisors.  As  soon  as  the  mouth  and 
nose  cavities  are  separated  and  as  fast  as  bone  is  formed  in  the 
jaws  most  of  the  space  is  occupied  by  the  tooth  germs. 


CHAPTER  XXVI. 
THE  DEVELOPMENT  OF  THE  TOOTH  GERM. 

The  Dental  Ridge. — By  the  middle  of  the  second  month  of  develop- 
ment the  arches  of  both  upper  and  lower  jaws  are  completed,  and  the 
palate  has  separated  the  nose  and  mouth  cavities.  The  first  indica- 
tion of  the  development  of  the  teeth  is  the  multiplication  of  the  cells 


. 


FIG.  270. — The  dental  ridge.  A  section  through  the  mandible  of  a  pig  embryo 
at  the  lower  edge,  two  spicules  of  bone  beginning  to  form;  to  the  right  Meckel's 
cartilage. 

of  the  epiblast  in  a  curved  line  on  the  crest  of  each  arch  in  the  area 
which  is  to  be  occupied  by  the  teeth.  By  this  multiplication  of 
cells  the  epiderm  is  piled  up  in  a  ridge,  projecting  above  the  surface, 
and  at  the  same  time  the  deep  layer  of  the  epiblast  is  forced  down 
into  the  underlying  mesoderm  (Fig.  270).  This  structure  is  known 
21  (321) 


322  THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

as  the  dental  ridge.  In  sections  the  cells  piled  up  above  the  surface 
are  usually  washed  off  more  or  less  by  the  reagents,  but  the  depres- 
sion into  the  mesoderm  is  shown.  On  the  lingual  surface  of  this 
ridge,  in  the  part  embedded  in  the  mesoderm,  the  cells  of  the  Mal- 
pighian  layer  grow  out  lingually  at  right  angles  to  the  ridge,  form- 
ing a  continuous  shelf  known  as  the  dental  lamina  (Fig.  271).  It 
is  important  to  remember  that  the  lamina  is  continuous  along  the 
entire  extent  of  the  ridge. 


FIG.  271. — The  dental  ridge  and  dental  lamina. 

The  Enamel  Organ.- — From  ten  points  on  the  surface  of  the  lamina 
little  buds  of  epiblast  start  and  grow  down  into  the  mesoderm, 
increasing  in  size  and  becoming  bulbous  at  the  deep  end.  The 
bulbous  portion  gradually  becomes  flattened.  At  this  stage  the 
bulb  is  composed  of  an  outer  layer  of  columnar  cells,  continuous 
with  the  Malpighian  layer  of  the  ridge  and  a  central  mass  of  large 
polyhedral  cells  (Fig.  272).  As  the  bud  continues  to  grow  into 
the  mesoderm,  the  mesodermic  tissue  below  it  begins  to  condense 
and  the  cells  of  the  upper  portion  of  the  bulb,  growing  more  rapidly, 
convert  the  bulb  into  a  two-layered  bag. 


THE  TOOTH  GERM 


323 


The  Dental  Papillae. — The  cells  in  the  condensed  mesoderm 
multiply  and  grow  up  into  the  cavity  of  this  cap,  forming  the  begin- 
hing  of  the  dental  papillae.  This  stage  is  represented  in  Figs.  273 
and  274,  in  which  the  enamel  organ  is  seen  connected  with  the 
lamina  by  a  cord  of  epithelial  cells,  and  made  up  of  an  outer  layer 
of  columnar  cells  known  as  the  outer  tunic,  and  an  inner  layer  of 
columnar  cells  lying  next  to  the  dental  papillae,  known  as  the  inner 
tunic.  The  polyhedral  cells  between  the  two  layers  fill  the  central 
part  of  the  enamel  organ  and  have  taken  on  a  peculiar  appearance, 
which  has  given  to  them  the  name  of  the  stellate  reticulum.  The 


FIG.  272. — A  section  through  the  mandibular  arch:  E,  enamel  organ;  D,  begin- 
ning of  the  dental  papilla;  B,  bone;  F,  fold  from  the  side  of  the  mandible  to  the  base 
of  the  tongue  covering  the  beginning  of  the  sublingual  gland;  T,  tongue. 


development  of  the  tooth  germ  now  progresses  until  the  dental 
papilla  has  taken  on  the  typical  form  of  the  tooth.  The  fully  formed 
enamel  organ  for  an  incisor  of  a  sheep  is  shown  in  Fig.  275.  The 
cord  which  connects  the  outer  tunic  with  the  surface  epithelium  is 
not  shown  in  this  section. 

The  Tooth  Germ. — The  tooth  germ  is  composed  of  the  enamel 
organ,  made  up  of  the  outer  tunic,  the  inner  tunic,  and  the  stellate 
reticulum,  covering  the  dental  papillae.  From  the  base  of  the 
papillee  fibrous  tissue  develops,  growing  upward  around  the  entire 
tooth  germ  and  enclosing  it  in  a  definite  wall  or  sac  of  fibrous  tissue. 
This  is  known  as  the  dental  follicle,  or  the  follicle  wall. 


324 


THE  DEVELOPMENT  OF  THE  TOOTH  GERM 


The  Dental  Follicle. — This  term  has  been  used  to  indicate  not 
simply  the  connective-tissue  wall,  but  all  of  the  structure  enclosed 
in  it.  This  use  of  the  term,  however,  is  confusing,  and  the  term 
should  be  confined  to  the  fibrous  sac.  By  the  end  of  the  twelfth 
week  the  follicle  wall  has  grown  up  so  as  to  enclose  the  enamel 


FIG.  273. — The  enamel  organ.    The  outer  tunic  connected  to  the  lamina  by  the  cord; 
the  dental  papilla  growing  up  into  the  cap.    The  spaces  are  shrinkage  spaces. 

organ,  and  the  epithelial  cord  which  has  connected  it  with  the 
surface  is  broken. 

Tooth  Germs  of  the  Permanent  Tooth. — Before  the  epithelial  cord 
is  broken,  from  some  point  on  the  lingual  surface  of  the  outer  tunic 
or  along  the  cord  a  bud  of  epithelial  cells  growls  out  and  turns 
down  into  the  mesoderm,  passing  over  the  follicle  wall  (Fig.  276). 
This  continues  to  grow  downward  until  it  has  reached  the  position 
below  and  to  the  lingual  of  the  tooth  germ  for  the  temporary  tooth, 


BEGINNING  OF  CALCIFICATION 


325 


where  it  develops  into  the  enamel  organ  for  the  corresponding  per- 
manent tooth.  It  goes  through  the  same  changes  of  form  as  has 
been  seen  in  the  temporary  teeth. 

Beginning  of  Calcification. — About  the  sixteenth  week  the  tooth 
germs  of  all  the  temporary  teeth  have  been  completely  enclosed 
in  their  follicles  and  the  enamel  organ  for  the  corresponding  per- 
manent teeth  have  begun  their  development  (Fig.  277).  This 
illustration  shows  a  section  through  the  lower  jaw  of  a  pig,  and 


FIG.  274. — The  enamel  organ,  a  little  older  than  Fig.  273.  It  shows  the  outer  tunic, 
the  inner  tunic,  and  the  stellate  reticulum.  The  dental  papilla  in  the  hollow  of 
the  cap.  The  spaces  are  caused  by  shrinkage. 


exhibits  the  tooth  germs  for  two  incisors  at  about  the  stage  of  the 
closing  of  the  follicle  walls.  The  buds  for  the  permanent  teeth  are 
seen  on  the  lingual,  and  the  formation  of  enamel  and  dentin  is 
just  beginning  in  the  temporary  teeth.  Notice  the  remains  of 
Meckel's  cartilage,  and  the  extension  of  endomembranous  bone 
formation  which  is  just  beginning  to  form  a  periosteum  on  its 
surface.  The  bone  has  grown  around  Meckel's  cartilage  and  around 
the  tooth  germs  on  the  buccal  and  lingual,  enclosing  them  in  an  open 


326 


THE  DEVELOPMENT  OF  THE  TOOTH  GERM 


groove,  which  will  later  be  completed  and  divided  into  separate 
crypts  for  each  tooth.  Fig.  278  is  from  a  similar  specimen  in  the 
region  of  a  temporary  molar.  The  dental  papilla  is  taking  on  the 
form  of  a  crown  and  the  formation  of  enamel  and  dentin  is  ready 
to  begin.  The  cells  on  the  outer  layer  of  the  dental  papilla  have 
developed  into  odontoblasts,  forming  a  single  layer  of  columnar 


FIG.  275. — The  tooth  germ,  from  the  mandible  of  a  sheep.  The  enamel  organ  shows 
the  outer  tunic,  inner  tunic,  and  stellate  reticulum.  The  dental  papilla  projects  into 
the  enamel  organ.  The  follicle  is  attached  to  the  base  of  the  dental  papilla  and 
surrounds  the  enamel  organ.  The  spicules  of  bone  form  the  crypt  wall. 


cells  lying  in  contact  with  the  inner  tunica  of  the  enamel  organ. 
Here  the  formation  of  enamel  and  dentin  begins,  the  dentin  slightly 
preceding  the  enamel.  The  odontoblasts  form  and  calcify  dentin 
matrix  from  without  inward.  The  cells  of  the  inner  tunic  or  amelo- 
blasts  form  and  calcify  the  enamel  rods  and  cementing  substance, 
progressing  from  within  outward.  The  line  upon  which  the  odonto- 


FIRST  PERMANENT  MOLAR 


327 


blasts  and  ameloblasts  lie  in  contact  therefore  will  become  the 
dento-enamel  junction.  The  formation  of  dentin  and  enamel  begin 
at  separate  points,  which  are  at  first  very  close  together,  but  are 


FIG.  276. — The  tooth  germ  showing  the  bud  for  the  permanent  tooth  at  P.  Cal, 
cification  is  just  beginning:  F,  follicle  wall;  D,  dental  papilla;  T,  inner  tunic;  T'- 
outer  tunic;  S,  stellate  reticulum;  O,  odontoblasts;  A,  ameloblasts;  B,  bone. 

carried  farther  apart  by  the  growth  of  the  dental  papilla,  until  they 
have  progressed  along  the  dento-enamel  junction  and  unite,  when 
the  increase  in  the  diameter  of  the  dental  papilla  is  stopped.  This, 
perhaps,  will  be  better  understood  by  studying  Figs.  82  to  87. 

First  Permanent  Molar. — The  origin  and  development  of  the  first 
permanent  molar  differs  from  that  of  all  the  other  permanent 


328  THE  DEVELOPMENT  OF  THE  TOOTH  GERM 


SM 

'V 


FIG.  277. — A  section  through  the  lower  jaw  of  a  embryo  pig,  showing  germs  of 

two  incisors. 


FIG.  278. — Germ  of  a  premolar  from  an  embryo  pig. 


FIRST  PERMANENT  MOLAR 


329 


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330  THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

teeth  in  important  respects.  It  is  the  only  permanent  tooth  whose 
enamel  organ  springs  directly  from  the  dental  lamina  in  the  same 
way  as  those  for  the  temporary  teeth.  It  is  the  only  permanent 
tooth  whose  crown  is  calcified  before  the  individual  is  thrown  upon 
its  own  resources  for  the  obtaining  of  nourishment.  Nature  seems 
to  have  taken  special  precautions  in  the  formation  of  this  most 
important  tooth. 

About  the  seventeenth  week,  at  a  point  on  the  dental  lamina, 
posterior  to  the  enamel  organs  of  the  temporary  teeth,  a  bud  starts 
to  grow  down  into  the  mesoderm,  which  develops  into  the  enamel 
organ  for  the  first  molar,  and  by  the  ninth  month  the  follicle  is 
complete  and  calcification  has  begun. 

The  Origin  of  the  Second  and  Third  Molars. — The  enamel  organ 
for  the  second  molar  is  formed  from  a  bud  given  off  from  the  outer 
tunic  of  the  enamel  organ  of  the  first  molar.  The  enamel  organ 
for  the  third  molar  is  formed  from  a  bud  given  off  from  the  outer 
tunic  of  the  enamel  organ  of  the  second,  at  about  the  third  year. 

Chronology. — The  development  of  the  teeth  was  first  investi- 
gated by  Lagros  and  Magitot  (about  1865).  Since  that  time  their 
work  has  been  repeated  and  verified  by  several  investigators.  About 
1880  Dr.  Black  repeated  the  entire  work  of  Magitot,  and  some  of 
his  illustrations  were  used  by  Dr.  Dean  in  his  Translation  of  Magi- 
tot Memoir.  Magitot's  table,  showing  the  chronology  of  tooth 
development,  is  given  on  page  329. 

The  previous  pages  are  to  be  considered  as  a  series  of  definitions, 
and  descriptions  of  structures,  and  now  the  student  is  assumed  to 
have  some  idea  of  what  is  meant  when  the  "dental  ridge,"  or  the 
"dental  papilla  "  is  mentioned. 

In  embryology  so  many  things  are  going  on  at  the  same  time  and 
the  changes  are  so  rapid  that  it  is  difficult,  especially  from  written 
description,  to  obtain  a  clear  idea  of  the  process.  Unfortunately 
a  moving  picture  of  the  development  of  the  tooth  cannot  be  made 
by  direct  photography  as  has  been  done  with  the  growth  of  plants 
and  the  opening  of  flowers,  but  it  is  important  to  visualize  the  process 
as  would  be  done  by  a  moving  picture,  the  present  description  is 
intended  to  connect  and  relate  in  a  most  elementary  way  some  of  the 
most  important  facts. 

The  First  Indication  of  Tooth  Development. — The  first  indication  of 
tooth  development  is  the  multiplication  of  epidermal  cells  about 
the  maxillary  and  mandibular  arches.  This  produces  a  cord  or  rod 
of  epiblastic  cells  projecting  above  the  surface  of  the  jaw  arch  and 


THE  FOLLICLE  WALL  331 

extending  into  the  mesoderm  of  the  body  of  the  arch.  The  extension 
into  the  mesoderm  is  more  or  less  vertical  to  the  surface  of  the  primi- 
tive jaw.  This  is  the  "dental  ridge."  On  the  lingual  side  of  this 
structure  the  epidermal  cells  grow  out  forming  a  layer  or  shelf  pro- 
jecting from  the  lingual  side  of  the  ridge  and  extending  as  far  as  the 
ridge  itself.  This  newgrowth  is  at  first  nearly  at  right  angles  to  the 
axis  of  the  ridge,  but  the  tip  of  it  turns  down  into  the  mesoderm, 
becoming  more  and  more  parallel  with  the  axis  of  the  original  dental 
ridge.  This  is  the  "dental  lamina." 

Early  in  the  development  of  the  lamina  at  ten  points  in  each  arch, 
epidermal  buds  start  from  the  edge  of  the  lamina  to  form  the  enamel 
organs  for  the  ten  temporary  teeth.  When  these  buds  start  they  are 
springing  from  the  edge  of  the  lamina,  but  after  the  formation  of  the 
enamel  organs  for  the  temporary  teeth  has  started  the  growth  of  the 
lamina  continues  growing  down  to  the  lingual  of  the  developing 
temporary  tooth  germs.  The  extent  and  continuity  of  this  develop- 
ment seems  to  be  different  in  different  species.  A  true  mental  picture 
of  this  process  will  explain  the  conflicting  statements  as  to  the  origin 
of  the  enamel  organs  for  the  permanent  teeth,  which  correspond  to 
or  replace  the  temporary  ones.  The  enamel  organs  for  these  teeth 
are  said  by  different  authors  (1)  to  arise  from  the  outer  tunic  of  the 
enamel  organ  of  the  temporary  tooth;  (2)  from  the  cord  connecting 
the  outer  tunic  of  the  temporary  tooth  with  the  surface  epithelium; 
(3)  or  direct  from  the  lamina. 

Enamel  Organ. — As  soon  as  the  enamel  organ  begins  to  grow  down 
into  the  mesoderm.  There  is  a  response  in  the  mesoderm  below  it 
resulting  in  a  change  in  the  character  of  the  cells,  and  the  develop- 
ment of  the  dental  papilla.  The  epithelial  cells  of  the  inner  tunic 
assume  the  form  of  ameloblasts  and  the  mesoblastic  cells  of  the  outer 
surface  of  the  dental  papilla  become  columnar  and  take  the  form  of 
odontoblasts.  This  specialization  begins  at  the  tip  of  the  dental 
papilla  and  at  the  points  that  will  be  the  beginnings  of  calcification. 
This  specialization  spreads  from  these  points  along  the  surface  of  the 
papilla.  The  formation  of  enamel  begins  while  the  enamel  organ  is 
still  in  its  typical  form,  that  is,  while  the  outer  tunic  is  complete 
and  is  still  connected  with  the  lamina  by  a  cord  of  epithelial  cells, 
but  almost  immediately  after  the  formation  of  enamel  and  dentin 
begins,  there  are  important  changes. 

The  Follicle  Wall. — As  soon  as  the  dental  papilla  and  enamel  organ 
begins  to  take  on  their  full  form,  there  occurs  differentiation  of  tissue 
in  the  mesoderm  and  the  formation  of  fibrous  tissue.  This  begins 


332  THE  DEVELOPMENT  OF  THE  TOOTH  GERM 

at  or  near  the  base  of  the  papilla,  but  rapidly  extends  upward 
(incisally)  passing  outside  of  the  outer  tunic  inclosing  both  structures 
in  a  fibrous  sac.  When  this  formation  of  fibrous  tissue  reaches  the 
incisal  extremity  of  the  enamel  organ  and  approaches  the  point 
from  which  the  cord  of  epithelial  cells  extends  to  the  lamina,  the  cord 
is  broken  and  the  enamel  organ  is  no  longer  connected  with  the 
surface.  At  this  time  four  important  things  happen :  (1)  The  begin- 
ning of  calcification  of  enamel  and  dentin;  (2)  the  breaking  up  of  the 
outer  tunic  of  the  enamel  organ  which  begins  at  the  point  where  the 
cord  was  broken;  (3)  a  marked  proliferation  of  epithelial  cords  and 
masses  arising  from  the  cells  which  formed  the  cord;  (4)  the  begin- 
ning of  the  bud  to  form  the  enamel  organ  for  the  successional 
tooth. 

The  Breaking  up  of  the  Outer  Tunic. — When  the  follicle  wall  closes 
over  the  incisal  extremity  of  the  enamel  organ,  there  appears  on 
the  outer  surface  of  the  outer  tunic  of  the  enamel  organ  little 
rounded  projections  of  epithelial  cells,  and  the  layer  is  broken  up. 
At  the  same  time  there  is  the  formation  of  capillary  bloodvessels 
from  the  follicle  wall,  which  carry  the  remains  of  the  outer  tunic 
down  against  the  inner  tunic  to  form  the  stratum  intermedium 
(Fig.  238).  There  is  an  intimate  relation  between  capillary  blood- 
vessels and  the  stratum  intermedium.  Leon  Williams  considered 
that  the  cells  of  this  layer  take  up  materials  from  the  blood  and 
elaborate  them  to  be  used  by  the  ameloblasts  in  the  calcification  of 
enamel.  Enamel  is  formed  only  as  far  as  the  stratum  intermedium 
is  formed,  although  the  inner  tunic  of  the  enamel  organ  extends 
apically  along  the  dental  papilla  toward  the  end  of  the  root  as  far  as 
dentin  is  formed. 

The  Breaking  up  of  the  Epithelial  Cord.— After  the  closing  of  the 
follicle  wall  the  cells  which  formed  the  cord  multiply  and  are  mixed 
with  fibrous  tissue.  This  is  no  longer  a  continuous  cord  of  epithelial 
cells,  but  irregular  strings  and  masses  of  epithelial  cells  lying  in  the 
fibrous  tissue.  This  has  been  called  the  cingulum,  extending  from 
the  follicle  wall  to  the  surface  epithelium. 

It  often  happens  that  the  epithelial  masses  take  on  globular 
form  and  it  is  probable  that  occasionally  one  of  these  may  develop 
into  an  enamel  organ,  and  lead  to  the  formation  of  a  supernumerary 
temporary  tooth. 

The  hud  for  the  corresponding  permanent  tooth  grows  downward 
(apically)  along  the  lingual  side  of  the  germ  of  the  temporary  tooth 
outside  of  its  follicle  wall,  until  it  conies  to  a  position  below  and  to  the 


ORIGIN  OF  THE  SECOND  AND  THIRD  MOLARS         333 

lingual  of  it,  where  it  goes  through  exactly  the  same  changes  that 
have  taken  place  in  the  development  of  the  temporary  one. 

At  the  time  the  follicle  wall  closes  over  the  enamel  of  the  per- 
manent tooth  there  occurs  a  similar,  but  usually  more  marked  and 
extensive  proliferation  of  epithelium,  and  the  origin  of  supernumer- 
ary permanent  teeth  is  so  explained.  The  supernumerary  would 
develop  between  the  temporary  and  the  permanent  tooth,  and  as  a 
rule  it  is  found  clinically  that  in  such  cases  the  first  tooth  to  erupt 
after  the  loss  of  the  temporary  one  is  the  supernumerary  and  the 
last  one  the  typical  tooth. 

Origin  of  the  Second  and  Third  Molars. — If  one  can  visualize  the 
process  that  has  been  described  it  will  be  realized  that  it  is  quite 
difficult  from  the  appearances  of  a  few  sections  to  determine  whether 
the  enamel  organs  for  the  second  and  third  molars  which  have  no 
temporary  predecessors  arise  from  a  bud  from  the  outer  tunic  of 
the  preceding  (approximating)  molar,  or  whether  there  is  an  exten- 
sion of  the  lamina  distally  from  which  the  buds  are  formed. 


CHAPTER  XXVII. 

THE  RELATION  OF  THE   TEETH  TO  THE   DEVELOP- 
MENT  OF  THE  FACE. 

AT  birth  the  jaws  contain  all  of  the  temporary  teeth  and  the 
first  molars  in  a  partially-formed  condition,  and  the  follicles  for 
all  of  the  permanent  teeth  except  the  second  and  third  molars. 
These  very  nearly  fill  the  substance  of  the  bone.  In  the  growth  of 
the  bones  of  the  face  and  the  changes  that  occur  in  the  transfor- 
mation of  the  child  to  the  adult  face,  the  teeth  play  a  most  important 
role. 

Before  considering  this  subject  in  detail  it  is  necessary  to  recall 
in  this  connection  some  things  that  have  already  been  emphasized. 

RELATION    OF    THE    TEETH    TO    THE    BONE. 

In  evolution  the  teeth  originally  had  no  connection  with  the 
bone,  it  being  formed  later  for  their  support.  In  embryology  the 
tooth  is  formed  first,  and  the  bone  formed  around  it.  In  this  way 
the  development  of  the  individual  repeats  evolution.  In  the  study 
of  the  bone  it  has  been  emphasized  that  the  connective  tissues 
have  been  specialized  to  meet  mechanical  conditions,  and  that 
both  ontogenetically  and  phylogenetically  they  are  formed  in 
response  to  mechanical  stimuli.  The  mutations  of  connective  tissue 
have  been  dwelt  upon,  and  especially  the  fact  that  a  bone  as  an 
organ  of  support  always  contains  fibrous  tissue,  and  that  there  is 
a  continual  oscillation  between  formation  and  destruction,  by  means 
of  which  it  is  perfectly  adapted  to  its  mechanical  environment. 
The  transformations  of  bone  in  bone  growth  have  been  pointed 
out,  and  these  will  be  still  more  carefully  studied  in  connection 
with  the  growth  of  the  bones  of  the  face. 

Some  years  ago  the  author  undertook  a  study  of  the  structure 
and  growth  of  the  jaws  and  alveolar  process,  which  resulted  in 
very  important  modifications  of  the  conceptions  of  the  matter  as 
given  by  standard  texts.  Tomes  describes  the  process  of  develop- 
ment as  essentially  an  addition  at  the  posterior  portions  of  the 
(334) 


RELATION  OF  THE  TEETH  TO  THE  BONE 


335 


jaws  to  make  room  for  the  successively  developed  permanent 
molars,  and  illustrates  the  process  in  diagrams  (Fig.  279). l  The 
following  quotation  states  his  view : 

"But  the  main  increase  in  the  size  of  the  jaw  has  been  in  the 
direction  of  backward  elongation;  in  this,  as  Kolliker  first  pointed 
out,  the  thick  articular  cartilage  plays  an  important  part.  The 
manner  in  which  the  jaw  is  formed  might  also  be  described  as  waste- 
ful; a  very  large  amount  of  bone  is  formed  which  is  subsequently, 
at  no  distant  date,  removed  again  by  absorption;  or  we  might 
compare  it  to  a  modelling  process,  in  which  thick,  comparatively 


FIG.  279. — Tomes'  diagram  of  development  of  mandible  from  infant  to  adult. 

shapeless  masses  are  dabbed  on  to  be  trimmed  and  pared  down 
into  form. 

"To  bring  it  more  clearly  home  to  the  student's  mind,  if  all  the 
bone  ever  formed  were  to  remain,  the  coronoid  process  would 
extend  from  the  condyle  to  the  region  of  the  first  bicuspid,  and 
all  the  teeth  behind  that  would  be  buried  in  its  base;  there  would 
be  no  neck  beneath  the  condyle,  but  the  internal  oblique  line 
would  be  a  thick  bar  corresponding  in  width  with  the  condyle. 
It  is  necessary  to  fully  realize  that  the  articular  surface  with  its 

1  Tomes'  Dental  Anatomy,  p.  208. 


336        THE   TEETH  AND  DEVELOPMENT  OF  THE  FACE 

cartilage  has  successively  occupied  every  spot  along  this  line; 
and  as  it  progresses  backward  by  the  deposition  of  fresh  bone  in 
its  cartilage,  it  had  been  followed  up  by  the  process  of  absorption, 
removing  all  that  was  redundant." 

In  a  similar  way  in  any  maxilla,  the  temporary  dentition  is  shown 
to  occupy  about  the  same  space  as  the  permanent  teeth,  as  far  as 
the  second  bicuspid,  and  the  adult  is  supposed  to  be  formed  from 
the  child  by  the  building  on  of  the  bone  at  the  back  as  the  molars 
are  formed. 

This  conception  is  fundamentally  misleading,  for  if  the  infant 
mandible  were  to  be  shown  in  the  relation  to  that  of  the  adult  in 
three  dimensions  of  space,  it  would  be  found  to  be  above  and 
entirely  within  the  adult  mandible,  and  no  part  of  the  bone  which 
constituted  the  infant  jaw  is  present  in  the  adult.  In  the  upper, 
if  the  temporary  teeth  at  two  years  were  figured  in  relation  to 
those  of  the  adult,  they  would  be  placed  somewhere  up  in  the  nasal 
cavity. 

The  conditions  are  more  correctly  stated  by  saying  that  forces 
exerted  at  the  posterior  portions  of  the  jaw  through  the  develop- 
ment of  the  successive  molars  cause  the  bone  to  grow  downward, 
forward,  and  outward  in  the  upper  arch,  upward,  forward,  and 
outward  in  the  lower,  carrying  the  bone  into  an  entirely  new  posi- 
tion in  space. 

In  this  process  the  peridental  membrane,  periosteum,  and 
articular  cartilage  all  play  their  part,  but  all  the  bone  posterior  to 
the  second  bicuspid  cannot  be  thought  of  as  having  been  formed 
by  the  articular  cartilage  and  modelled  into  form  by  the  periosteum, 
as  might  be  inferred  from  Tomes'  statement. 

Structure  of  Maxillae  and  Mandible. — Before  attempting  to  follow 
the  growth  of  the  bone  in  the  development  of  the  face,  the  arrange- 
ment and  distribution  of  the  varieties  of  bone  in  the  structure  of 
the  mandible  and  maxillse  should  be  carefully  studied. 

Cortical  Plate. — The  outer  surface  of  these  bones  is  formed  of 
a  compact  layer  composed  partly  of  subperiosteal  and  partly  of 
Haversian  system  bone.  This  varies  greatly  in  thickness,  depend- 
ing upon  the  stress  to  be  sustained.  It  is  called  the  cortical  plate. 

Cancellous  Bone. — The  center  of  the  bone  is  cancellous  in  charac- 
ter and  made  up  of  thin  plates  of  lamella?  arranged  around  large 
medullary  spaces.  The  direction  and  arrangement  of  these  plates 
is  determined  by  the  forces  received  on  the  cortical  plates  and  the 
directions  of  stress  to  which  they  are  subjected.  This  was  pointed 


RELATION  OF  THE  TEETH  TO  THE  BONE      337 

out  some  years  ago  by  Walkoff  in  an  elaborate  study  of  the  bones 
by  the  use  of  the  z-rays.  By  this  means  he  showed  that  the  plates 
of  cancellous  bone  in  certain  areas  had  a  definite  arrangement  which 
was  related  to  the  attachments  of  certain  muscles.  From  the 
examination  of  sections  of  the  mandible  it  will  be  found  that  not 
only  is  the  general  form  of  the  bone  determined  by  the  forces  to 
which  it  has  been  subjected,  but  also  that  its  minute  inner  structure 
is  definitely  arranged  with  reference  to  these  forces.  The  direction 
and  arrangement  of  the  plates  of  cancellous  bone  are  continually 


FIG.  280. — The  distribution  of  bone  in  the  alveolar  process. 

changed  and  rebuilt  to  readjust  them  to  the  support  of  new  condi- 
tions (Fig.  326). 

Cribriform  Plates. — The  alveoli  or  sockets  into  which  the  roots 
of  the  teeth  fit  are  bounded  by  a  thin,  definite  wall,  which  is  pierced 
by  a  great  many  openings.  These  have  been  called  the  cribriform 
plates,  or  sieve-like  plates.  They  unite  the  cortical  plates  of  the 
bone  at  the  border  of  the  alveolar  process,  and  are  fused  with  it, 
on  their  labial  and  lingual  sides.  The  cribriform  plates  forming 
the  walls  of  the  alveoli  are  really  made  up  of  a  thin  layer  of  sub- 
22 


338        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

peridental  bone,  which  has  been  built  on  to  the  plates  of  cancellous 
bone,  to  attach  the  fibers  of  the  peridental  membrane  (Fig.  213). 
Within  the  substance  of  the  bone  and  surrounding  the  course  of 
the  inferior  dental  artery  and  nerve  is  found  what  Cryer  has  called 
the  cribriform  tube.  This  extends  from  the  point  where  the  arteries 
and  vein  enter  the  substance  of  the  bone  on  the  lingual  surface  of 
the  ramus,  posterior  to  the  alveolar  process  and  below  the  oblique 
line,  and  extends  through  the  cancellous  portion  of  the  body  of 
the  bone,  emerging  at  the  mental  foramina.  It  is  really  a  rather 
definite  arrangement  of  the  plates  of  cancellous  bone  around  the 
vessels  and  the  nerves. 


FIG.  281. — Skull  of  orang-outang. 

Alveolar  Process. — If  the  adult  alveolar  process  as  seen  in  the 
skull  is  examined,  it  is  apparent  that  the  bone  is  arranged  so  as 
to  give  the  greatest  support  with  the  least  possible  bulk,  and  where 
there  is  an  increase  in  bulk  it  is  to  meet  some  special  force  (Fig. 
280).  The  incisors  and  cuspids  are  used  chiefly  to  bite  off  pieces 
of  food,  and  when  the  food  cannot  be  bitten  it  is  torn  and  wrenched 
away.  This  puts  a  heavy  strain  in  all  directions  on  the  roots  of 
the  teeth,  which  must  be  supported  by  the  bone.  For  this  reason 
the  roots  of  the  incisors  are  usually  well  covered  with  bone  through 
their  entire  length.  The  cuspid  root  is  long  and  the  upper  portion 
of  it  so  wrell  supported  in  the  bone  at  the  side  of  the  nose  and  toward 
the  orbit  that  the  most  convex  portion  of  it  is  sometimes  uncovered. 
In  animals  that  use  the  incisors  largely  for  tearing,  wrenching,  and 
fighting,  the  bone  is  greatly  thickened  over  the  incisal  roots,  as  is 
shown  in  the  skull  of  the  orang-outang  (Fig.  281). 


RELATION  OF  THE  TEETH  TO  THE  BONE 


339 


In  the  upper  molars  the  spreading  of  the  three  roots  gives  abun- 
dant support  against  the  direct  forces  of  occlusion.  The  grinding 
motions  bring  lateral  pressure  against  the  inclined  planes  of  the 
cusps,  which  is  met  by  a  thickening  of  the  process  in  its  occlusal 


FIGS.  282  and  283. — Human  mandible,  showing  form  of  the  bone  and  the  positions 
from  which  sections  were  cut. 

third  (Fig.  280),  forming  a  heavier  ring  of  bone,  while  the  buccal 
roots  are  often  exposed  in  their  middle  third.  In  the  molars  the 
buccal  incline  of  the  lingual  cusps  of  the  upper  occlude  with  the 
lingual  incline  of  the  buccal  cusps  of  the  lower  when  the  jaws  are 
brought  squarely  together,  and  in  the  grinding  motions  the  outward 
pressure  on  the  lower  molars  is  supported  by  the  great  mass  of  the 


FIG.  284. — Human  mandible,  showing  form  of  the  bone  and  the  positions  from 
which  sections  were  cut. 

body  of  the  bone,  while  the  inward  pressure  is  supported  by  a 
thickening  of  the  occlusal  third,  as  the  entire  alveolar  process 
projects  lingually  from  the  body  of  the  bone.  In  the  examination 
of  any  collection  of  skulls,  the  amount  and  arrangement  of  the 


340        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 


bone  of  the  alveolar  process  will  be  found  to  be  an  indication  of 
the  masticatory  habits  of  the  individual. 

In  examining  the  sections  through  the  bone  of  the  alveolar 
process,  the  adaptation  of  the  arrangement  of  bone  to  the  force 
to  be  sustained  should  be  constantly  kept  in  mind. 

Influence  of  Mechanical  Conditions  in  Evolution. — Professor  E.  D. 
Cope,1  in  a  long  treatise  on  "The  Mechanical  Causes  of  the  Develop- 
ment of  the  Hard  Parts  in  Mammals,"  has 
elaborated  the  fact  that  the  bones  of  the 
skeletons  of  all  mammals  have  been  influ- 
enced in  their  development  by  mechanical 
conditions,  and  that  their  present  forms 
are  adaptations  to  physical  environment. 
In  this  he  states,  as  a  general  principle 
of  structure,  that  the  bone  is  most  dense, 
but  least  in  amount,  on  the  side  in  the 
direction  toward  which  forces  have  been 
exerted  in  development,  and  less  dense, 
but  greater  in  amount,  on  the  sides  from 
which  the  forces  have  been  exerted. 
These  statements  should  be  applied  in 
the  study  of  all  the  sections  shown. 

An  old  dry  mandible  was  sawed  through 
in  the  positions  indicated  in  the  illustra- 
tion (Figs.  282,  283,  and  284). 

The  portion  containing  the  bicuspid 
and  molar  on  the  left  side  was  ground 
through  the  molar  to  obtain  a  section 
parallel  with  the  axis  of  the  tooth.  The 
portion  between  the  alveolus  of  the 
cuspid  and  second  bicuspid  on  the  left 
side  was  ground  vertically  through  the 
area  where  the  first  bicuspid  had  been 
(Fig.  285).  The  portion  on  the  right 

side  containing  the  two  bicuspids  and  molar  was  ground  to  give 
three  sections  at  right  angles  to  the  roots — one  in  the  gingival 
third,  one  about  the  middle  of  the  root,  and  one  just  at  their  apices 
(Fig.  286).  The  distal  portions  of  the  bone  were  decalcified  and 
sections  cut  through  the  alveoli  of  the  second  and  third  molars 
(Figs.  287  and  288). 

1  Journal  of  Morphology,  1888. 


FIG.  285. — Ground  sec- 
tion through  the  mandible 
where  the  bicuspid  had 
been  extracted. 


RELATION  OF  THE  TEETH  TO  THE  BONE      341 

The  Distribution  of  Bone  in  the  Mandible. — In  Chapter  XVII,  on 
Bone,  it  was  stated  that  the  arrangement  of  the  layers  in  the  tissue 
could  be  read  as  a  record  of  the  manner  of  formation.  In  the  exam- 
ination of  these  sections  the  arrangement  of  the  lamella?  is  to  be 


FIG.  286. — Transverse  sections  through  the  roots  of  two  bicuspids  and  the  first 
molar,  showing  distribution  of  bone. 

studied  in  this  way,  as  well  as  the  distribution  of  the  varieties  of 
bone.  Where  the  bicuspid  had  been  extracted  the  alveolus  has 
been  filled  with  fairly  compact  bone,  rounding  over  the  border  of 
the  process.  The  section  ground  through  this  position  shows  the 


342  THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 


FIG.  287. — Decalcified  sections  through  the  molar  region. 


RELATION  OF  THE  TEETH  TO  THE  BONE 


343 


FIG.  288. — Decalcified  sections  through  the  alveoli  of  the  second  and  third  molars. 


344    THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

buccal  and  lingual  cortical  plates  in  U  shape.  The  two  plates  are 
braced  together  across  the  central  portion  by  spicules  of  cancellous 
bone.  At  the  occlusal  border  the  outline  of  the  old  alveolus  can 
still  be  seen  by  studying  the  section  carefully  with  the  microscope. 
After  the  extraction  of  the  tooth  the  socket  was  first  filled  with 
connective  tissue,  which  was  later  transformed  into  bone,  joining 


FIG.  289. — A  section  ground  through  the  first  molar. 

that  of  the  alveolar  wall.  Near  the  lower  border,  the  subperi- 
osteal  bone  is  found  to  be  very  thick,  the  bone  evidently  grow- 
ing in  that  direction.  Near  the  occlusal  border  on  the  lingual 
side,  there  have  evidently  been  absorptions  of  the  surface,  removing 
the  Ilaversian  system  bone,  and  then  a  few  layers  of  subperiosteal 
bone  have  been  reformed  on  the  surface. 


RELATION  OF  THE  TEETH  TO  THE  BONE 


345 


Fig.  289  shows  a  section  ground  through  the  molar.  The  cribri- 
form plates  lining  the  alveoli  join  the  cortical  plates  at  the  border 
of  the  process.  On  the  lingual  side  the  wall  of  the  process  is  very 


FIG.  290. — The  buccal  pk 


FIG.  291. — The  lingual  plate  from  Fig.  286. 

thin,  but  is  thickened  in  the  occlusal  third  to  support  the  tooth 
against  force  exerted  lingually.  On  the  buccal  side  the  cribriform 
plate  of  the  alveolar  wall  is  connected  with  the  cortical  plate  by 


346        THE  TEETH  AND  DEVELOPMENT  OF  ^THE  FACE 

spicules  of  cancellous  bone.  Below  the  apex  of  the  root  the  cortical 
plates  are  connected  by  cancellous  bone  in  which  the  medullary 
spaces  are  much  larger.  The  same  arrangement  of  the  cortical 
plate  and  its  bracing  is  shown  in  Fig.  287,  which  cuts  between  the 
alveoli  of  the  second  and  third  molar.  Fig.  327  and  Plate  XIX 
should  be  studied  in  this  connection,  remembering  that  the  bone 
has  been  formed  and  shaped  by  formation  of  subperiosteal  bone  on 
its  surface  and  subperidental  bone  at  the  border  of  the  process  and 
their  transformation  into  Haversian  system  and  cancellous  bone. 

Fig.  286  is  cut  transversely.  Notice  that  the  gingival  section 
has  been  turned  over  in  mounting.  Observe  the  cribriform  plates 
forming  the  walls  of  the  alveoli,  and  the  way  these  are  braced 
against  each  other  and  the  cortical  plates  by  bands  of  cancellous 


FIG.  292. — The  bone  between  the  alveoli  of  the  mesial  and  distal  roots  of  the  first 
molar,  from  Fig.  286. 

bone.  In  accordance  with  the  principles  noted,  the  buccal  plate 
is  thin  and  very  compact,  while  the  lingual  plate  is  much  thicker, 
but  more  open  in  structure,  and  the  direction  of  growth  has  been 
toward  the  buccal  as  the  arch  of  the  jaw  increased  in  size.  Fig. 
290  shows  the  buccal  plate  with  higher  magnifications,  Fig.  291 
the  lingual  plate,  and  Fig.  292  the  bone  separating  the  alveoli 
from  the  mesial  and  distal  roots  of  the  molar.  The  third  figure  of 
this  series  shows  only  the  tip  of  the  distal  root  of  the  molar,  but  the 
arrangement  of  plates  of  cancellous  bone  between  the  cortical 
plates  is  nicely  shown. 

The  Maxilla. — In  the  maxilla  the  arrangement  is  exactly  on  the 
same  plan,  the  details  being  different  because  of  the  difference  in 
the  shape  of  the  bone. 


THE  GROWTH  OF  THE  JAWS  347 

THE   GROWTH   OF   THE   JAWS. 

It  has  long  been  noted  that  at  birth  the  mandible  is  straight, 
and  with  the  eruption  of  the  teeth  the  ramus  develops  and  the  body 
increases  in  size.  In  this  process  the  thickness  of  the  bone  is 
increased  from  the  mental  foramina  to  the  alveolar  border,  and  the 
body  of  the  bone  approaches  a  right  angle  with  the  ramus.  When 
the  teeth  are  lost  or  lose  their  function  the  alveolar  process  is 
destroyed  and  the  bone  reduced  in  thickness  from  above  downward 
until  the  mental  foramen  comes  to  lie  on  the  upper  surface  of  the 
bone.  The  mandible  performs  two  functions,  a  respiratory  and  a 
masticatory  function,  and  it  should  be  remembered  that  these 
are  influential  in  its  development.  The  object  of  this  section  is 
to  give  some  conception  of  the  direction  of  growth  in  the  devel- 


FIG.  293.— Skull  at  birth. 

opment  of  the  bones  of  the  face  and  the  way  in  which  the  changes 
are  brought  about. 

This  can  best  be  done  by  studying  the  series  of  skulls  from  child- 
hood to  old  age,  in  which  the  outer  cortical  plate  has  been  removed 
so  as  to  show  the  developing  teeth  in  their  crypts  and  the  relation 
of  the  forming  teeth  to  those  already  in  occlusion  (Figs.  293  to 
307).  At  birth  all  of  the  teeth  except  the  second  and  third  molars 
have  begun  to  develop,  and  their  tooth  germs  are  lying  embedded 
in  the  cancellous  substance  of  the  maxillae.  In  the  upper  jaw  they 
occupy  almost  all  of  the  space  to  the  floor  of  the  nose  and  orbit, 
and  there  is  little  if  any  indication  of  the  maxillary  sinus  (Fig.  293) 


348        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

Each  tooth  germ  is  enclosed  in  a  separate  crypt,  the  wall  of  which 
is  formed  by  a  cribriform  plate.    The  walls  of  the  crypts  are  braced 


FIG.  294. — Maxillae  at  about  eight  months  after  birth,  showing  the  unerupted 

tooth. 


FIG.  295. — Maxillae  at  about  one  year. 


against  each  other  and  the  cortical  plates  of  the  maxillse  by  spicules 
of  cancellous  bone  surrounding  medullary  spaces.     As  the  tooth 


THE  GROWTH  OF  THE  JAWS  349 

develops  within  its  crypt,  pressure  is  exerted  and  the  crypt  wall 
is  pushed  backward  through  the  cancellous  bone. 

Growth  Force. — The  force  exerted  by  the  growing  tooth  is  the 
result  of  the  multiplication  of  cells  in  the  tooth  germ,  and  is  exactly 
comparable  to  the  forces  exerted  by  multiplication  of  cells  in  any 
position.  For  instance,  the  force  exerted  by  the  multiplication  of 
the  cells  in  a  rootlet  of  a  plant  is  sufficient  to  force  pebbles  aside 
and  make  an  opening  through  hard  packed  earth.  Some  attempts 
have  been  made  to  measure  the  amount  of  force,  but  we  can  only 
say  that  it  appears  to  be  considerable,  acting  through  short  range. 


FIG.  296. — Maxillae  at  one  and  one-half  years. 

How  this  force  is  generated  has  been  a  matter  of  much  speculation 
and  investigation.  It  shows  some  points  of  similarity  with  the 
swelling  of  wood  fibers  when  water  is  added.  It  apparently  is 
related  to  osmosis,  and  has  some  direct  relations  to  blood-pressure. 
It  is  certainly  a  very  complicated  matter,  with  chemical  affinities 
at  the  bottom  of  it. 

Forces  Influencing  Bone  Growth. — While  the  growing  tooth  germs 
are  producing  force  which  causes  conditions  of  stress  of  the  cortical 
plates,  the  growth  of  the  tissues  within  the  mouth — the  tongue 
and  the  associated  organs — is  exerting  pressure  upon  the  lingual 


350        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

surfaces  of  the  bone.  The  muscles  attached  to  their  surfaces  trans- 
mit force  to  the  bone  through  the  periosteum,  and  the  functions 
of  mastication,  deglutition,  and  respiration  are  acting  upon  them. 
All  of  these  are  mechanical  stimuli,  to  which  the  connective-tissue 
cells  respond.  In  all  the  process  of  development  the  growth  is 
the  result  of  all  the  forces  to  which  the  bones  are  subjected,  per- 
fectly distributed  through  the  substance  of  the  bone  by  the  agency 
of  normal  occlusion.  Any  lack  of  harmony  in  the  proportion  of 


FIG.  297. — Maxillae  in  the  second  year,  showing  the  relation  of  the  erupting  teeth. 
Note  the  relation  of  the  crypt  of  the  second  molar  to  the  inferior  dental  canal. 

these  forces  may  allow  the  teeth  to  meet,  when  they  erupt,  outside 
of  the  normal  influence  of  their  cusps,  causing  the  beginning  of 
malocclusion.  Any  malocclusion  disturbs  the  balance  in  the 
distribution  of  forces,  and  results  in  a  disturbance  of  the  develop- 
ment of  bone,  which  progresses  during  the  entire  period  of  devel- 
opment. This  must  result  in  the  lack  of  balance  in  the  proportions 
of  the  features  which  will  be  proportionate  to  the  malocclusion. 
It  has  been  natural  and  almost  inevitable,  because  of  their  hard- 


THE  GROWTH  OF  THE  JAWS 


351 


FIG.  298. — The  complete  temporary  dentition  (about  three  years),  showing  the  rela- 
tion of  the  developing  permanent  teeth. 


FIG.  299. — The  complete  temporary  dentition  and  the  first  permanent  molar.    Note 
the  relation  of  the  bicuspids  to  the  temporary  molars.     (In  the  seventh  year.) 


352        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

ness,  to  think  of  bones  as  solid  and  unchanging.  In  the  study  of 
these  skulls  the  bones  of  the  face  must  be  viewed  not  as  solid  and 
rigid,  but  as  containing  millions  of  active  cells  which  are  continually 
building  and  rebuilding  their  substance. 


FIG.  300. — Front  view  of  the  skull  shown  in  Fig.  299.    Note  the  relation  of  the  per- 
manent incisors  and  cuspids  to  each  other  and  the  roots  of  the  temporary  teeth. 

Usually  somewhere  between  the  seventh  and  ninth  months  after 
birth  the  growth  of  the  central  incisors  causes  the  absorption  of 
the  roof  of  their  crypts,  and  the  tooth  moves  occlusally,  cutting 
through  the  soft  tissues  (Fig.  294).  The  formation  of  cementum 
on  the  surface  of  the  root  and  of  bone  on  the  wall  of  the  crypt 
attach  the  connective-tissue  fibers  and  form  the  beginning  of  the 


THE  GROWTH  OF  THE  JAWS  353 

peridental  membrane.  As  the  tooth  moves  occlusally  the  bone 
grows  up  around  it  from  the  circumference  of  the  crypt  wall,  con- 
verting it  into  the  wall  of  the  alveolus.  The  root  is  not  fully  formed 
and  the  conical  pulp  filling  the  funnel-like  end  exerts  force  by  the 


FIG.  301. — Dentition  in  the  eighth  year.     Note  the  position  of  the  cuspids  and  com- 
pare with  Fig.  303. 


multiplication  of  cells  and  the  blood-pressure,  which  cause  the 
tooth  to  move  occlusally  and  the  bone  to  grow  in  that  direction. 
At  the  same  time  the  pressure  of  tongue  and  lips  exerts  pressure  on 
the  surfaces  of  the  tooth  and  bone,  influencing  the  direction  of 
bone  growth.  The  jaw  increases  in  thickness  in  the  occlusal  direc- 
23 


354        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

tion  and  grows  forward  and  outward.  At  the  same  time  the  growth 
'of  each  successively  distal  tooth  is  exerting  pressure  upon  those 
already  erupted,  causing  them  to  move  farther  in  the  occlusal 
direction.  In  Figs.  296  and  297  notice  the  way  in  which  the  crypt 
walls  are  pushed  downward  by  the  development  of  the  tooth  root 


FIG.  302. — The  left  side  of  the  skull,  shown  in  Fig.  301. 

until  the  inferior  dental  nerve  lies  between  the  floor  of  the  crypt 
and  the  cortical  plate  of  the  lower  border.  In  this  way  enough 
pressure  may  be  produced  to  cause  reflex  nervous  symptoms,  which 
commonly  precede  the  eruption  of  the  temporary  molars,  and  so 
development  continues  until  all  of  the  temporary  teeth  are  in 


THE  GROWTH  OF  THE  JAWS  355 

place.  About  the  sixth  year  the  first  permanent  molars  take  their 
place  at  the  distal  of  the  temporary  teeth  and  their  cusps  interlock 
(Fig.  299).  The  importance  of  these  teeth  can  scarcely  be,  over- 
stated. They  are  not  only  to  be  the  chief  means  of  mastication 


FIG.  303. — Dentition  in  the  eleventh  year.    Note  the  growth  of  the  cuspids  and 
bicuspids.    The  second  molar  is  about  to  erupt. 


during  the  period  in  which  the  temporary  teeth  are  lost  and  replaced 
by  their  successors,  but  they  are  to  maintain  the  relation  of  the 
jaws  to  each  other.  The  way  in  which  these  teeth  lock  determines 
the  balance  between  the  forces  exerted  by  the  action  of  the  muscles 


356        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

attached  in  the  region  of  the  ramus,  and  those  in  the  region  of  the 
symphysis  (Fig.  299). 

A  deviation  from  the  normal  relation  of  these  teeth  will  entirely 
change  the  direction  of  the  forces,  and  will  be  manifested  by  a 
modification  in  the  development  in  the  bone.  In  the  skull  at  this 
period  the  bicuspids  are  seen  lying  below  the  temporary  molars, 
and  the  second  molar  developing  at  the  distal  of  the  first.  Their 


FIG.  304. — Dentition  in  the  thirteenth  year.    Note  the  relation  of  the  bicuspid 
crown  to  the  roots  of  the  lower  temporary  molar. 


growth  is  transmitted  through  the  teeth  to  the  alveolar  process, 
and  the  addition  of  bone  results.  The  same  skull  viewed  from  in 
front  (Fig.  300)  shows  the  relation  of  the  permanent  incisors  and 
cuspids  to  the  temporary  ones.  In  the  lower  jaw  the  temporary 
centrals  have  been  lost  and  the  permanent  ones  are  forcing  their 
way  between  the  temporary  laterals.  The  crowns  of  the  centrals 
are  wider  than  those  of  the  teeth  that  were  lost,  and  they  conse- 


THE  GROWTH  OF  THE  JAWS  357 

quently  exert  pressure  upon  the  mesial  surfaces  of  the  laterals, 
pushing  them  apart  and  carrying  them  upward  and  forward. 

Study  the  relation  of  the  lower  centrals,  laterals,  and  cuspids 
in  the  development  of  the  arches  at  from  six  to  ten  years.  Notice 
that  the  roots  of  the  central  are  not  fully  formed,  that  the  lateral 


FIG.  305. — The  dentition  of  a  young  adult.    The  third  molars  have  not  erupted. 
(About  fifteen  years.) 

lies  to  the  lingual  of  the  temporary  lateral  root,  and  with  its  mesio- 
occlusal  angle  below  the  distal  surface  of  the  central.  The  develop- 
ment of  the  cuspid  has  pushed  the  crypt  floor  through  the  cancellous 
bone  until  it  has  reached  the  solid  cortical  plate,  and  still  the  forma- 
tion of  the  crown  is  not  quite  completed.  The  six  teeth  form  a 


358        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 


FIG.  306. — Adult  dentition.     Note  the  distance  from  the  apices  of  the  incisors  to 
the  lower  border  of  the  mandible  and  the  floor  of  the  nose. 


FIG.  307. — Edentulous  jaws,  showing  loss  of  alveolar  process. 


THE  GROWTH  OF  THE  JAWS  359 

triangle  of  which  the  centrals  are  the  apex,  and  the  cortical  plates 
from  cuspid  to  cuspid  the  base.    The  completion  of  the  roots  of 


FIGS    308  and  309  were  photographed  in  the  same  relative  size,  to  show  the  amount        \^ 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


these  teeth  will  carry  the  temporary  teeth,  alveolar  process  and 
all,  upward,  forward,  and  outward,  thus  increasing  the  distance 


360        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

from  the  mental  foramen  to  the  symphysis  and  enlarging  the  arc 
of  the  jaw  from  cuspid  to  cuspid. 


FIGS.  310  and  311  were  photographed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


In  the  same  skull  notice  the  relation  of  the  upper  incisors  and 
cuspids  to  the  corresponding  temporary  teeth.  They  lie  to  the 
lingual  of  the  roots  of  the  temporary  teeth,  the  lateral  a  little  to 


THE  GROWTH  OF  THE  JAWS 


361 


FKJS.  312  and  313  were  photographed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


362     THE  TEETH  AND  DEVELOPMEXT'OF  THE  PACK 


FIGS.  314  and  315  were  photographed  in  the  same  relative  size,  to  show  the  amount 
and  direction  of  growth,  with  the  development  of  the  full  permanent  dentition. 


363 

the  lingual  of  the  central  and  cuspid.  The  cuspid  has  pushed 
back  the  floor  of  its  crypt  until  it  is  braced  against  the  solid  bone 
at  the  base  of  the  malar  process.  The  growth  of  these  teeth  will 
first  cause  the  temporary  teeth  to  move  occlusally,  the  bone  grow- 
ing from  the  border  of  the  process  to  follow  them.  In  this  growth 
the  distance  from  cuspid  to  cuspid  is  increased  and  spaces  appear 
between  the  temporary  incisors  some  time  before  they  are  lost. 

If  such  spaces  do  not  appear,  the  development  is  not  progress- 
ing normally,  and  artificial  force  should  be  applied  to  stimulate 
bone  growth.  If  this  is  not  done  the  permanent  teeth  are  sure  to 
come  in  more  or  less  rotated  and  out  of  position. 

In  Figs.  301  and  302  the  incisors  have  been  pushed  off  and  the 
permanent  ones  are  beginning  to  move  occlusally.  Notice  the 
relation  of  the  floor  of  the  crypt  to  the  floor  of  the  nose,  and  the 
root  has  scarcely  begun  to  develop.  In  the  adult  skull  (Fig.  306) 
there  is  almost  as  much  space  from  the  apex  of  the  root  to  the  floor 
of  the  nose  as  there  is  now  from  the  border  of  the  alveolar  process 
to  the  floor  of  the  nose.  The  result  of  the  growth  of  the  cuspids' 
roots  is  shown  by  comparing  Figs.  300  and  301  with  Fig.  303. 

The  Importance  of  Proximal  Contact. — The  proper  contact  of  the 
teeth  upon  their  proximal  surfaces  is  necessary  for  this  develop- 
ment. If,  for  instance,  the  mesial  angle  of  the  lower  lateral  fails 
to  engage  with  the  distal  surface  of  the  central,  but  slips  by  to  the 
lingual,  the  growth  of  the  cuspid  will  push  it  farther  and  farther 
past  the  central  instead  of  enlarging  the  arch.  One  of  the  cogs  in 
the  mechanism  has  slipped,  and  the  growth  of  bone  cannot  later  be 
expected  to  make  room  for  the  crowded  teeth. 

In  the  next  stage  of  growth  the  increase  in  size  is  from  the  mental 
foramen  to  the  ramus,  and  is  largely  influenced  by  the  development 
of  the  roots  of  the  bicuspids  and  the  second  molars.  Figs.  302 
and  303  show  the  relation  of  the  second  molar  to  the  distal  surface 
of  the  first,  and  it  will  be  seen  that  its  growTth  exerts  force  upon  the 
first  molar,  and  this  is  transmitted  through  the  arch  by  means  of 
proximal  contact.  Notice  the  inclination  of  the  bicuspid  roots, 
which  help  to  carry  the  growth  in  the  same  direction. 

After  the  second  molar  is  in  place  the  growth  of  the  third  should 
exert  the  same  force  and  room  be  provided  for  it  (Fig.  304).  The 
muscular  action  of  the  lips  and  tongue  are  specially  important 
in  these  last  stages  of  growth,  and  particularly  the  forces  that  are 
generated  by  the  action  of  the  muscles  in  respiration  and  deglutition. 
The  activity  of  the  connective-tissue  cells  in  the  bone  requires 


364        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

mechanical  stimuli  for  their  maintenance,  and  as  the  muscular 
action  is  vigorous  or  deficient,  the  growth  of  bone  will  be  full  and 
normal  or  imperfect  and  unbalanced.  It  appears  often  that  the 
bone  activity  becomes  so  sluggish  that  the  growth  of  the  third 
molar  cannot  produce  the  effect  it  should,  and  it  remains  impacted. 
A  comparison  of  figures  will  show  that  while  room  has  been  made 
for  the  third  molar,  all  of  the  upper  teeth  have  moved  downward, 
forward,  and  outward,  and  the  lower  ones  upward,  forward,  and 
outward.  Compare  the  distance  from  the  apex  of  the  incisor 


FIG.  316. — Two  years. 


FIG.  317. — Three  years. 


FIG.  318.— Six  years. 


FIG.  319.— Ten  years. 


Maxillae  photographed  from  the  median  line  in  the  same  relative  size,  to  show  the 
amount  and  direction  of  growth. 


THE  GROWTH  OF  THE  JAWS 


365 


FIG.  320. — Twelve  years. 


FIG.  321.— Adult. 


Maxillae  photographed  from  the  median  line  in  the  same  relative  size,  to  show  the 
amount  and  direction  of  growth. 


FIG.  322. — Bone  from  the  buccal  plate  of  the  mandible  of  a  young  sheep,  showing 
transformations  of  bone:  1,  subperiosteal  bone;  2,  Haversian  system  bone;  3, 
Haversian  system  bone  becoming  cancellous. 


366        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

roots  to  the  floor  of  the  nose  and  the  lower  border  of  the  mandible 
in  Figs.  305  and  306. 


FIG.  323.- — The  record  in  the  arrangement  of  the  lamellae  of  the  growth  of  the  man- 
dibles.   A  decalcified  section  from  near  the  lower  border  of  a  human  mandible. 


This  process  may  be  more  fully  realized  by  comparing  the  front 
views  of  the  skulls  (Figs.  308  to  315).    They  were  all  photographed 


THE  GROWTH  OF  THE  JAWS 


367 


with  the  same  lens  and  bellows  length,  so  as  to  make  the  pictures 
of  the  same  relative  size  as  the  skulls.  Notice  the  increase  in  dis- 
tance from  the  floor  of  the  nose  and  the  floor  of  the  orbit  to  the 
edges  of  the  upper  incisors,  and  from  the  lower  border  of  the  man- 
dible to  the  edge  of  the  lower  incisors.  It  will  be  seen  that  if  the 
infant  mandible  were  placed  in  relation  to  the  adult  mandible  it 
would  lie  entirely  within  the  arch  and  in  the  mouth  cavity,  while 


FiG.  324. — A  decalcified  section  from  the  lingual  vertical  plate  of  a  human  mandible, 
showing  the  arrangement  of  lamellae  as  a  record  of  growth. 


in  the  upper  the  temporary  incisors  in  Fig.  315  would  be  some  place 
in  the  nasal  cavity.  In  all  of  this  growth  the  size  of  the  air  spaces 
increases  with  the  movements  of  the  teeth,  the  floor  of  the  nose 
and  palate  growing  downward  and  developing.  This  may  be 
shown  in  Figs.  316  to  321,  in  which  the  right  half  of  the  maxilla 
has  been  removed  from  dissected  skulls  and  photographed  from 
the  median  line. 


368        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

Tissue  Changes  in  the  Physiologic  Movements  of  the-  Teeth  — All 
that  has  been  said  in  regard  to  bone  growth  must  be  recalled  in 


FIG.  325. — Cancellous  bone  from  a  decalcified  section  of  a  human  mandible,  showing 
reconstructions  to  change  the  direction  of  the  spicules. 

order  to  obtain  a  conception  of  the  manner  in  which  these  move- 
ments of  the  teeth  and  the  development  of  the  bone  are  accom- 
plished. Bone  laid  down  under  the  periosteum  and  the  peridental 


THE  GROWTH  OF  THE  JAWS 


369 


membrane  has  been  transformed  into  Haversian  system  bone  and 
then  made  cancellous,  as  illustrated  in  Fig.  322,  which  is  taken 


FIG.  326. — Decalcified  section  of  cancellous  bone  from  a  human  mandible,  showing 
absorptions  and  rebuildings,  changing  the  direction  of  the  spicules. 

from  the  buccal  plate  of  the  mandible  of  a  young  sheep.    Reversed 
changes  have  also  been  going  on,  the  periosteum  cutting  into  the 
24 


370        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

Haversian  bone  by  absorption  and  the  cancellous  bone  being  con- 
densed into  Haversian  system  bone.  These  changes  leave  a  record 
in  the  arrangement  of  the  lamellae,  and  may  be  studied  in  decalcified 
sections  (Figs.  323  to  326).  Even  the  direction  of  the  spicules  of 


FIG.  327. — A  longitudinal  section  through  the  tip  of  the  alveolar  process  of  a  tem- 
porary tooth  about  ready  to  be  lost:  D,  dentin;  Cm,  cementum,  showing  absorption 
and  rebuilding;  Pd,  peridental  membrane;  B,  bone  growing  occlusally  at  the  border 
of  the  process;  Hb,  rebuilt  Haversian  system  bone. 


cancellous  bone  are  being  constantly  changed  by  absorptions  and 
rebuilding  to  adjust  them  to  changes  of  stress. 

While  the  temporary  teeth  are  moving  occlusally,  bone  is  laid 
down  under  the  peridental  membrane  at  the  border  of  the  alveolar 
process,  which  is  at  once  cut  out  by  absorptions  and  replaced  by 


THE  GROWTH  OF  THE  JAWS 


371 


Haversian  system  bone  (Fig.  213).     The  alveolar  process  becomes 
a   veritable  patchwork,   as  shown  in  Figs.   327   and   328.     The 


FIG.  328. — A  longitudinal  section  through  the  temporary  alveolar  process,  which 
is  growing  occlusally  to  follow  the  temporary  tooth.  It  is  from  the  same  series  as 
Fig.  327,  but  shows  more  of  the  bone.  Study  the  absorptions  and  rebuildings,  as 
shown  in  the  arrangement  and  character  of  the  lamella?.  Pd,  peridental  membrane; 
Po,  periosteum. 


372        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

permanent   tooth  developing  in  its  crypt  produces  conditions  of 
pressure,  and  osteoclasts  appear  in  all  the  medullary  spaces,  around 


and  above  the  crypt,  and  through  the  alveolar  process,  as  well  as 
on  the  crypt  wall.    They  are  more  active  in  the  medullary  spaces, 


THE  GROWTH  OF  THE  JAWS 


373 


cutting  away  the  spicules  of  bone,  thinning  and  cutting  apart  the 
crypt  wall,  and  allowing  it  to  be  bent  and  pushed  back. 


2  5 


Fig.  329  shows  the  alveolar  process  on  the  lingual  side  of  the 
temporary  incisor,  and  illustrates  the  enlargement  of  the  medullary 
spaces  preparatory  to  the  eruption  of  the  permanent  tooth.  Fig. 


374        THE  TEETH  AND  DEVELOPMENT  OF  THE  FACE 

330  shows  the  labial  plate  of  the  process,  and  notice  that  the  bone  is 
being  formed  under  the  periosteum  and  at  spots  under  the  peridental 
membrane,  while  the  substance  of  the  bone  is  being  destroyed. 


a  o  jj 


When  the  tooth  is  finally  pushed  off  from  the  gum  all  but  a  few 
bits  of  the  alveolar  process  have  been  destroyed,  and  as  the  per- 


THE  GROWTH  OF  THE  JAWS  375 

manent  tooth  comes  through,  bone  formation  begins  at  the  border, 
patching  on  to  the  remains  of  the  old  process  (Fig.  331).    r- 

In  studying  the  absorption  of  bone  around  the  crypt  walls,  it 
has  been  noted  that  the  osteoclasts  appear  first  in  the  cancellous 
bone  (Figs.  214  and  215),  surrounding  the  crypts  and  outside  of 
it.  Absorptions  here  remove  the  spicules  which  brace  the  crypt 
wall,  and  cut  through  the  wall  in  such  a  way  as  to  allow  it  to  be 
pushed  back  through  the  weakened  substance.  In  the  same  way 
in  the  movements  of  the  teeth,  absorptions  appear  first  in  the 
spaces  outside  of  the  cribriform  plates  of  the  alveoli,  until  the 
remaining  bone  is  weakened  sufficiently  to  spring  under  the  press- 
ure. All  of  the  sections  of  the  mandible  should  be  studied  as  a 
recoid  of  these  bone  transformations,  and  especially  in  orthodontia 
it  should  be  remembered  that  appliances  are  used  not  to  push  the 
teeth  through  the  bone  as  a  post  would  be  pushed  through  the 
mud,  but  to  supply  mechanical  stimuli  to  living  cells  whose  activity 
will  result  in  bone  growth,  carrying  the  teeth  into  their  proper 
positions,  and  finally,  that  teeth  will  remain  only  in  the  position  in 
which  all  of  the  forces  to  which  it  is  subjected  are  balanced. 


PART  II. 

DIRECTIONS  FOR  LABORATORY  WORK. 

(TWENTY-FIVE  PERIODS  IN  THE  LABORATORY  ) 


PRELIMINARY. 

IT  is  assumed  in  this  work  that  the  student  has  had  a  course  in 
general  histology,  including  laboratory  work,  that  he  is  familiar 
with  the  technique  of  handling  the  microscope,  the  technique  of 
staining  and  mounting  sections,  and  that  he  is  able  to  recognize 
at  once  the  elementary  tissues.  The  same  outfit  is  required  as  for 
general  histology,  including  slides  and  blank  labels  for  them; 
cover-glasses;  teasing  needles;  forceps;  section  lifter;  a  tube  of 
balsam;  a  funnel;  pipette;  filter  paper  and  lens  paper;  6  one-ounce 
reagent  bottles  containing  xylol,  absolute,  95,  and  70  per  cent, 
alcohols,  hematoxylin,  and  eosin;  at  least  two  chip  butter  dishes 
that  can  be  used  for  staining;  a  box  for  the  slides;  a  note-book;  a 
hard  and  a  soft  drawing  pencil;  a  good  eraser;  and  a  piece  of  clean 
soft  linen  for  wiping  slides  and  cover-glasses. 

Teeth  for  Grinding. — It  is  difficult  to  obtain  satisfactory  teeth 
for  the  grinding  of  microscopic  sections,  and  the  student  should 
bring  to  the  laboratory  a  number  of  suitable  teeth  from  which 
selection  can  be  made.  Old,  dry  teeth  are  absolutely  useless  for 
the  purpose,  however  perfect  their  structure  may  have  been.  When 
a  tooth  has  been  extracted  for  some  time  the  tissues  dry  out,  giving 
up  a  considerable  amount  of  water,  and  consequently  shrink. 
The  shrinkage  of  dentin  and  enamel  is  unequal,  and  the  result  is 
a  cracking  of  the  tissue.  The  observation  of  the  teeth  in  any  skul 
will  reveal  cracks  in  the  enamel  that  may  be  seen  with  the  naked 
eye,  the  tooth  often  splitting  lengthwise.  Besides  the  cracks  that 
can  be  seen,  the  tissue  is  full  of  microscopic  cracks.  When  the 
grinding  of  sections  from  such  teeth  is  attempted,  before  the  sec- 
tion is  reduced  to  sufficient  thinness  for  microscopic  observation, 

(377) 


378  DIRECTIONS  FOR  LABORATORY  WORK 

the  enamel  will  break  to  pieces  and  be  lost.  A  tooth  that  is  to  be 
used  for  grinding  must  be  placed  in  solution  as  soon  as  it  is  extracted, 
and  never  at  any  stage  of  the  process  be  allowed  to  dry,  until  ready  for 
mounting.  Any  solution  that  will  prevent  decomposition  will  do 
for  this  purpose.  The  best  that  I  have  found  is  a  4  per  cent,  formal- 
dehyde in  50  per  cent,  alcohol.  This  may  be  roughly  prepared  by 
diluting  95  per  cent,  alcohol  with  an  equal  volume  of  water  and 
adding  one  part  of  formalin  to  nine  parts  of  the  diluted  alcohol : 

Alcohol 45  c.c. 

Water 45  c.c. 

Formalin 9  c.c. 

This  solution  not  only  prevents  the  drying,  but  has  a  hardening 
action  on  the  organic  matter,  which  facilitates  the  grinding.  Teeth 
may  be  preserved  in  this  indefinitely. 

Teeth  Required. —From  his  collection  the  student  should  select 
for  grinding  an  incisor  or  cuspid,  a  bicuspid,  and  a  molar.  The 
teeth  should  be  free  from  caries  and  their  crowns  as  perfect  as 
possible. 

The  Relation  of  the  Section  to  the  Crown. — The  practical  value  of 
the  study  of  ground  sections  depends  upon  obtaining  from  them  a 
knowledge  of  enamel-rod  directions  in  relation  to  the  tooth  crown 
as  well  as  the  section.  In  operating  the  teeth  are  looked  at  from 
their  outside  surface,  but  the  operator  needs  to  see  in  the  enamel 
not  simply  a  hard  and  extremely  dense  tissue,  but  a  tissue  made  up 
of  minute  rods  whose  general  direction  he  knows  beforehand.  If 
a  tooth  is  selected  and  a  section  cut  from  it  in  a  knowrn  position, 
and  the  relation  of  the  section  to  the  crown  remembered,  the  direc- 
tion of  enamel  rods  can  be  placed  in  relation  to  the  entire  crowrn  as 
well  as  to  the  section.  This  is  one  of  the  objects  to  be  sought  in 
the  making  of  the  outline  drawings. 

Location  of  the  Section. — Having  selected  the  teeth  for  grinding, 
the  next  step  is  to  locate  the  position  and  direction  of  the  section. 
This  must  be  so  placed  as  to  cut  the  enamel  rods  in  their  length. 
The  section  from  the  incisor  or  cuspid  should  be  ground  labiolin- 
gually,  but  the  section  from  the  molar  and  bicuspid  may  be  ground 
either  buccolingually,  mesiodistally,  or  diagonally.  The  surface 
of  the  tooth  should  be  considered,  and  the  section  placed  in  an 
area  in  which  the  student  desires  to  discover  the  enamel-rod  direc- 
tions and  the  structure  of  the  tissue.  The  line  of  the  section  should 
now  be  marked  on  the  tooth  with  India  ink  and  a  fine  pen. 


PREPARATION  OF  GROUND  SECTIONS  OF  TEETH   379 

The  Drawings  of  the  Teeth. — After  marking  the  position  of  the 
section  the  tooth  should  be  carefully  and  accurately  drawn,  showing 
the  position  of  the  section  as  seen  from  the  axial  and  occlusal 
surfaces. 

Grinding  of  the  Section. — Every  institution  should  have  a  machine 
for  the  preparation  of  ground  sections,  but  such  a  machine  is  too 
delicate  an  instrument  to  be  handled  by  students.  In  the  appendix 
will  be  found  a  chapter  written  by  Dr.  Black  describing  the  grinding 
machine  and  the  technique  of  its  use.  If  one  is  available,  the  student 
may  have  his  sections  ground  for  him  and  returned  ready  to  mount, 
or  he  may  grind  them  himself,  using  the  following  technique: 

Preparation  of  Ground  Sections  of  the  Teeth.— For  this  work  the 
student  should  have  two  large  corundum  stones  not  less  than  four 
inches  in  diameter,  one  of  "C"  and  one  of  "E"  grit.  Corundum 
is  very  much  better  than  carborundum  for  this  purpose.  In  grind- 
ing the  stone  should  be  kept  revolving  slowly  and  moistened  with 
a  stream  of  water.  Holding  the  tooth  against  the  flat  side  of  the 
coarse  stone  with  the  fingers,  the  tissues  should  be  rapidly  ground 
away  until  the  position  marked  for  the  section  is  reached,  when  the 
fine  stone  should  be  substituted  and  the  grinding  continued  just 
enough  to  remove  the  scratches.  The  surface  should  now  be 
polished  on  the  Arkansas  stone  until  a  very  perfect  surface  has 
been  obtained.  Wash  the  specimen  clean  and  immerse  in  several 
changes  of  95  per  cent,  alcohol,  and  leave  in  absolute  alcohol  in 
a  closed  bottle  for  several  hours  or  over  night.  Harden  a  drop  of 
balsam  on  the  center  of  a  clean  slide  by  warming  it  over  a  Bunsen 
burner  to  evaporate  the  xylol.  When  the  slide  is  cool  the  balsam 
should  be  neither  sticky  nor  brittle.  Now  remove  the  tooth  from 
the  alcohol,  wipe  it  dry,  and,  placing  it  on  the  balsam  with  the 
polished  surface  next  to  the  glass,  gently  warm  the  slide  until  the 
balsam  is  thoroughly  softened,  and  press  the  tooth  down  against 
the  glass  and  clamp  it  firmly  in  position,  using  a  spring  clip.  Set 
it  away  to  harden  thoroughly,  when  the  grinding  may  be  continued. 

Holding  the  slide  parallel  with  the  surface  of  the  coarse  stone, 
the  tissues  may  be  rapidly  removed  until  the  section  is  about  as 
thin  as  a  calling  card,  when  the  fine  stone  should  be  substituted 
and  the  section  reduced  to  the  required  thinness.  It  should  not  be 
more  than  twenty  microns  in  thickness.  In  the  final  stages  progress 
of  the  grinding  may  be  followed  with  a  hand  magnifying  glass. 
Finally  the  surface  should  be  polished  on  an  Arkansas  stone.  The 
specimen  should  now  be  washed  with  alcohol,  the  balsam  removed 


380  DIRECTIONS  FOR  LABORATORY  WORK 

with  xylol,  and  brought  to  the  laboratory  in  95  per  cent,  alcohol, 
where  it  is  to  be  etched  and  mounted  according  to  the  directions. 

Every  step  in  the  above  technique  is  important  and  must  be 
followed  with  minute  care  and  accuracy.  Not  least  important  is 
the  cleaning  of  the  slide.  It  sometimes  happens  that  the  section 
will  be  loosened  from  the  glass  before  the  grinding  is  completed. 
This  is  usually  due  to  some  fault  in  the  technique.  When  it  happens 
it  is  best  to  finish  the  grinding  without  attempting  to  refasten  the 
section  to  the  slide.  To  do  this  the  section  should  be  held  against 
the  flat  side  of  the  stone,  using  a  fine-grained  cork,  a  piece  of  box- 
wood, or  some  similar  material.  The  danger  of  breaking  the  sec- 
tion, however,  is  much  greater. 

The  Preparation  of  Transverse  Sections  of  the  Root. — For  this 
purpose  one  of  the  flattened  roots  furnishes  the  best  material,  as, 
for  instance,  the  mesial  root  of  a  lower  molar,  the  root  of  a  lower 
bicuspid,  or  of  an  upper  second  bicuspid.  Holding  the  root  in  a 
vise  by  the  remains  of  the  crown,  with  a  metal  saw,  saw  off  the 
tip  of  the  root,  removing  an  eighth  of  an  inch  or  less.  Then  saw  off 
as  thin  a  slice  as  possible.  In  the  same  way  saw  out  at  least  two 
other  sections,  one  from  the  gingival  and  one  from  the  middle  third 
of  the  root.  These  should  be  dropped  into  a  bottle  of  formalin- 
alcohol  until  the  grinding  is  completed.  The  grinding  is  easily 
accomplished  on  the  flat  side  of  the  corundum  stone,  holding  the 
section  on  the  finger  or  under  a  cork.  The  last  grinding  should  be 
done  on  the  fine  Arkansas  stone. 

Transverse  sections  of  the  root  are  easily  ground  and  can  be 
made  very  thin. 

Manner  of  Working  in  the  Laboratory. — In  no  place  in  the  world 
can  time  be  wasted  more  easily  than  in  the  histological  laboratory. 
The  student  should  take  the  attitude  of  an  original  investigator 
and  study  out  the  material  for  himself  as  far  as  possible,  remember- 
ing that  he  has  a  far  better  opportunity  than  the  man  who  worked 
out  the  details  of  these  structures.  He  must  constantly  try  to 
picture  the  structure,  and  imagine  how  it  would  appear  if  sectioned 
in  another  direction. 

Drawings. — Drawings  from  the  microscope  are  made  not  simply 
to  occupy  the  student's  time,  nor  as  a  record  of  what  he  has  done, 
but  to  make  observation  more  accurate  and  detailed,  and  to  fix 
the  impressions  of  structure  more  perfectly  in  mind.  Many  stu- 
dents excuse  themselves  for  careless  and  slovenly  work  by  saying 
that  they  are  not  artists.  Anyone  without  any  knowledge  of  the 


USE  OF  DIRECTIONS  FOR  LABORATORY  WORK          381 

principles  of  art  can  in  a  very  short  time  acquire  the  ability  to  make 
excellent  microscopic  drawings.  A  few  principles  of  procedure  will 
help  greatly.  The  first  of  all  is  that  a  light  line  can  always  be 
made  darker,  therefore  the  drawing  should  always  be  kept  light 
until  the  later  stages. 

After  selecting  a  field,  draw  lightly  the  outline  of  the  principal 
masses  and  then  the  outlines  of  the  smaller  ones.  In  this  way  the 
proportion  of  objects  in  the  field  and  their  relation  to  each  other 
can  be  maintained.  Never  draw  any  detail  such  as  individual 
cells,  nuclei,  etc.,  until  all  of  the  outlines  are  completed.  Then 
work  in  the  details  in  the  darker  colored  areas.  The  making  of 
the  outlines  is  by  far  the  most  important  stage  in  the  drawings. 

Each  outfit  should  contain  a  6  H  and  an  H-B  pencil  and  a  good 
eraser,  which  must  be  kept  clean.  The  pencils  should  be  kept 
sharp  and  always  used  with  a  light  touch  upon  the  paper.  The 
beginner  always  tends  to  start  his  drawing  by  making  a  circle. 
This  should  be  avoided,  for  it  is  objects  that  are  being  studied, 
not  fields,  and  in  many  cases  the  object  cannot  be  bounded  by  a 
circle.  There  is  also  a  tendency  to  represent  the  object  smaller 
on  the  paper  than  it  appears  in  the  field. 

The  prime  qualities  in  a  microscopic  drawing  are  accuracy  and 
correctness  of  detail.  The  drawings  are  made  to  show  all  the  detail 
of  structure  that  can  be  observed.  It  often  happens  that  a  draw- 
ing that  looks  very  well  shows  very  little  knowledge  of  the  structure 
of  the  tissue  which  it  represents. 

Stencilled  Laboratory  Notes. — In  fifteen  years  of  teaching  the 
author  has  found  stencilled  notes  on  the  daily  work  in  the  labora- 
tory of  very  great  assistance.  There  are  always  variations  in  the 
appearance  of  the  material  which  cannot  be  anticipated  before 
the  sections  are  cut.  Very  often  something  will  be  seen  unusually 
well  that  would  not  be  mentioned  in  the  text-book.  Different 
stains  may  have  been  used  which  would  change  the  appearance  of 
the  tissues,  and  for  all  of  these  things  and  many  others  daily  notes 
are  very  convenient. 


USE    OF   DIRECTIONS   FOR   LABORATORY   WORK. 

At  the  beginning  of  the  laboratory  period  the  first  thing  to  be 
done  is  to  read  through  the  directions  for  the  day's  work.  The  amount 
of  work  for  the  day  is  then  clearly  in  mind,  and  all  the  steps  in  any 


382  DIRECTIONS  FOR  LABORATORY  WORK 

procedure  that  is  to  be  undertaken  are  .understood  at  the  begin- 
ning. It  is  necessary  to  divide  the  time  available,  so  as  to  accom- 
plish the  work  indicated  for  the  day. 

PERIOD   I. 

Drawings  of  Tooth  Surfaces  Showing  the  Position  of  Sections. — 

The  object  of  these  drawings  is  to  show  the  relation  of  the  section 
to  the  crown  from  which  it  is  ground,  so  that  in  studying  the 
enamel-rod  directions  as  seen  in  the  sections,  they  may  be  referred 
to  the  entire  crown.  The  drawings  should  be  made  from  five  to 
ten  times  natural  size,  and  must  be  made  accurately  to  scale  (Fig. 
332).  Measure  the  length  and  the  breadth  of  the  tooth  and  lay 
out  a  rectangle,  say  eight  times  these  dimensions,  to  serve  as  a 
guide  in  drawing.  If  the  tooth  is  marked  for  a  buccolingual  section, 
stick  the  apex  of  the  root  on  a  bit  of  wax  and  place  the  tooth  on 
the  table  with  the  buccal  surface  toward  you.  Do  not  change  its 
position  until  the  drawing  is  completed,  for  to  do  so  would  change 
lights  and  shadows.  After  getting  the  outline  accurately,  work 
in  the  shadows  so  as  to  give  the  drawing  roundness.  Remember 
in  doing  this  that  you  can  always  make  it  darker,  but  you  cannot 
erase  without  injuring  the  neatness  of  the  drawing.  When  the 
drawings  are  completed  the  section  is  ready  for  grinding,  which 
must  be  done  outside  of  the  laboratory,  following  the  directions 
in  Introduction  to  Part  II. 

PERIOD   H. 

Etching  and  Mounting  of  Ground  Sections. — At  the  desk  will  be 
found  1  per  cent,  hydrochloric  acid,  dilute  ammonia,  and  vaseline, 
which  are  the  only  reagents  not  included  in  the  outfit  and  required 
for  this  work.  The  sections  are  brought  to  the  laboratory  ground 
and  ready  to  mount.  Fill  one  of  the  dishes  with  water  and  carefully 
wash  the  specimen  free  from  all  debris  of  grinding.  Dry  the  sec- 
tion between  filter  papers,  so  as  to  remove  all  moisture  from  the 
surface.  Fill  one  dish  with  1  per  cent,  hydrochloric  acid,  and  the 
other  with  dilute  ammonia.  Put  a  very  little  vaseline  upon  the 
tip  of  the  finger,  and  holding  the  section  by  the  root  portion,  cover 
one  surface  of  the  crown  portion  with  a  very  thin  layer  of  it.  In 
doing  this  the  vaseline  should  be  wiped  from  the  center  toward 
the  edges  of  the  section,  so  as  to  prevent  it  from  running  over  on 


ETCHING  AND  MOUNTING  OF  GROUND  SECTIONS      383 


to  the  other  surface.  The  vaseline  is  to  confine  the  action  of  the 
acid  to  one  surface  of  the  enamel.  Holding  the  section  by  the 
root  portion,  immerse  the  crown  in  the  dilute  acid  for  thirty  seconds, 

DISTAL  SURFACE 


BUCCAL 


OCCLUSAL  SURFACE 
BUCCAL.    MARGIN 


DISTAL  M. 


MESIAL  M. 


LING 


LINGUAL  M. 


8  DIAMETERS 


FIG.  332. — Drawing  of  occlusal  and  axial  surfaces  of  a  tooth  to  show  the  relation 
of  the  section  to  the  tooth.     (Drawn  by  W.  A.  Offil,  1910.) 

or  until  minute  bubbles  can  be  seen  forming  upon  the  surface. 
Remove  and  immerse  at  once  in  the  dilute  ammonia  for  a  minute. 
Remove  the  vaseline  by  carefully  wiping  the  section  with  absolute 
alcohol  or  ether,  and  immerse  in  95  per  cent,  alcohol.  In  this  it 


384  DIRECTIONS  FOR  LABORATORY  WORK 

should  remain  while  the  slide  and  cover-glass  are  being  prepared. 
Obtain  from  the  desk  a  cover-glass  long  enough  to  cover  the  entire 
section  and  carefully  clean  both  slide  and  cover-glass.  On  the 
center  of  the  slide  place  a  drop  of  balsam  that  is  as  long  as  the 
section.  Holding  the  slide  over  a  Bunsen  burner  or  alcohol  flame, 
warm  it  gently  so  as  to  evaporate  the  xylol.  In  this  process  the 
drop  will  spread  out  over  the  slide  and  the  direction  of  spreading 
may  be  guided  by  the  heat.  Allow  the  slide  to  cool  and  test  the 
hardness  of  the  balsam  with  a  teasing  needle  or  the  finger-nail. 
When  cold  the  balsam  should  be  just  soft  enough  to  take  the 
imprint  of  the  needle  or  nail,  but  not  be  sticky.  If  it  is  sticky  it 
must  be  reheated;  if,  on  the  other  hand,  it  is  brittle  enough  to 
chip,  it  must  be  scraped  off  from  the  slide  and  the  process  tried 
again.  In  the  same  way  prepare  a  film  of  balsam  on  the  cover- 
glass.  Remove  the  section  from  the  95  per  cent,  alcohol  and  dry 
it  for  a  few  minutes  in  the  air  (after  wiping  with  filter  paper). 
Place  the  section,  etched  side  up,  upon  the  balsam  on  the  slide, 
and  place  the  cover-glass  on  it,  balsam  side  down.  Warm  the  slide 
gently  over  the  flame,  while  pressing  the  cover-glass  down  with 
the  handle  of  a  teasing  needle.  As  the  balsam  is  wrarmed,  the 
slide  and  cover-glass  are  brought  together,  forcing  the  balsam 
out  to  the  edge  of  the  cover-glass  in  all  directions.  All  excessive 
balsam  should  be  squeezed  out  at  the  edges.  Place  on  the  cover- 
glass  a  small  piece  of  blotting  paper  or  a  layer  of  cork,  adjust  some 
sort  of  a  spring  clip  and  put  the  section  away  until  the  balsam  is 
entirely  hard.  When  the  balsam  is  entirely  hard  the  excess  may  be 
removed  by  gently  scraping  with  a  knife-blade  and  wiping  with 
xylol.  The  section  should  now  be  labelled  writh  the  name  of  the 
tooth,  the  direction  and  position  of  the  section,  the  student's  name 
and  number,  and  the  date. 

The  mounting  in  hard  balsam  greatly  improves  the  value  of  the 
section,  for  the  dentinal  tubules  and  the  lacunae  of  the  cementum 
are  left  filled  with  air  and  can  be  more  easily  studied.  Sections 
may,  ho\vever,  be  mounted  in  the  ordinary  way,  in  soft  balsam. 
If  the  section  is  broken  or  extremely  thin,  soft  balsam  should  be 
used. 

PERIOD   HI. 

Outline  Drawings  from  Ground  Sections. — The  object  of  the  out- 
line drawing  is  the  study  of  the  dental  tissues,  their  distribution, 
portion  of  the  tooth  formed  by  each,  their  relation  to  each  other, 


OUTLINE  DRAWINGS  FROM  GROUND  SECTIONS         385 

and  the  coarser  points  of  their  structure.  To  get  the  value  from 
this  work  the  drawings  must  be  made  very  accurately  to  scale  and 
as  large  as  the  note-book  page  will  allow.  With  the  Boley  gauge 
or  a  millimeter  rule  measure  accurately  the  length  of  the  section, 
multiply  this  by  eight  or  ten,  and  mark  the  length  on  a  page  of  the 
drawing  book.  Measure  the  width  of  the  section  at  the  point  of 
the  greatest  diameter  and  multiply  this  by  the  same  factor.  Using 
this  for  the  width  and  the  previous  measurement  for  the  length, 
lightly  draw  a  rectangle,  which  is  to  be  used  as  a  guide  in  the  con- 
struction of  the  drawing.  The  success  of  the  drawing  now  depends 
on  the  accuracy  and  number  of  the  measurements. 

First  measure  the  vertical  distance  from  the  incisal  edge  to  the 
gingival  line  on  one  side  of  the  section,  and  then  on  the  other,  and 
mark  these  on  the  sides  of  the  rectangle.  This  will  give  the  rela- 
tive length  of  root  and  crown  and  the  difference,  if  any,  in  position 
of  the  gingival  line  on  the  two  sides.  Measure  the  vertical  dis- 
tance from  the  most  prominent  point  on  the  axial  surface  to  the 
incisal  edge  or  the  tips  of  the  cusps,  and  so  on,  making  every 
measurement  that  can  help  in  the  formation  of  the  drawing.  In 
this  way  the  outline  of  the  section  should  first  be  traced  inside  the 
rectangle,  then  the  dento-enamel  junction,  then  the  pulp  chamber 
is  shown,  and  finally  the  cementum.  Before  drawing  the  outline 
of  the  cementum,  the  section  should  be  placed  under  the  micro- 
scope, using  the  low  power,  and  the  cementum  should  be  observed, 
studying  it  from  the  gingival  line  on  one  side  of  the  section  to  the 
gingival  line  on  the  other. 

It  would  be  a  waste  of  time  to  attempt  to  fill  in  the  structure  of 
the  tissue  of  the  entire  outline,  and  only  certain  things  are  to  be 
shown  in  these  drawings.  For  that  reason  fill  in  three  portions  of 
enamel  and  dentin  and  three  portions  of  cementum  and  dentin, 
using  the  low-power  objective.  Study  first  the  bands  of  Retzius 
(page  68),  and  lightly  indicate  their  direction.  Study  the  enamel- 
rod  direction,  beginning  at  the  gingival  line  at  one  side  and  follow- 
ing it  around  the  crown  to  the  other  side.  In  a  portion  at  the 
incisal  edge,  or  on  the  occlusal  surface,  indicate  the  rod  directions, 
and  in  the  same  way  show  them  in  a  portion  near  the  center  of 
the  axial  surface  on  one  side  and  near  the  gingival  line.  Follow 
the  dentinal  tubules  which  end  next  to  the  portions  of  enamel 
which  have  been  filled  in  to  the  point  where  they  open  into  the 
pulp  chamber,  and  indicate  their  direction  (page  139).  In  the  same 
way  fill  in  three  portions  of  the  cementum  and  the  dentin  under 
25 


386 


DIRECTIONS  FOR  LABORATORY  WORK 


them — one  in  the  gingival  line,  one  near  the  middle  of  the  root, 
and  one  in  the  region  of  the  apex  (Fig.  333). 


ENAMEL 
--.ENAMEL  RODS 

'-DENTINE  (TUBULES} 


-PULP  CHAMBER 


—  CEMENTUM 


FIG.  333. — Outline  drawing  of  longitudinal  section,  made  as  a  study  of  the  dental 
tissue.     (Drawn  by  E.  J.  Schmidt.) 


If  any  portion  of  the  section  has  been  lost  in  grinding,  that 
portion  should  be  indicated  by  dotted  lines,  and  in  the  same  way, 
if  a  portion  of  the  crown  has  been  lost  by  wear,  the  original  form 
may  be  added  in  dotted  lines. 


MINUTE  STUDY  &F  .ENAMEL  AND  DENTIN  387 

Outline  drawings  should  be  made  from  each,  of  the  three  classes 
of  teeth — one  from  the  incisor  or  cuspid,  one  from  a  bicuspid, 
and  one  from  a  molar,  and  a  laboratory  period  should  be  devoted 
to  each  drawing. 

PERIOD   IV. 

Isolated  Enamel  Rods. — Obtain  from  the  desk  a  fragment  of 
enamel  which  has  been  broken  in  the  direction  of  the  rods.  Place 
a  drop  of  distilled  water  or  glycerin  on  the  center  of  a  clean  slide. 
Moisten  the  broken  surface  with  a  drop  of  water  and  lightly  scrape 
it  with  the  blade  of  a  broad,  sharp,  chisel,  holding  the  edge  parallel 
with  the  surface  and  the  shaft  at  right  angles  to  it.  Dip  the  edge 
of  the  chisel  in  the  drop  of  liquid  on  the  slide,  and  the  scrapings 
will  be  left.  Cover  with  a  cover-glass  and  study  with  the  high 
power,  using  a  small  diaphragm.  Fragments  of  enamel  will  be 
found  made  up  of  broken  rods,  some  single  and  others  in  groups. 
Note  the  diameter  of  the  rods  and  the  appearance  of  the  cross- 
markings,  which  will  be  seen  if  the  light  is  properly  adjusted. 
Draw  as  seen  with  the  high  power. 

Repeat  this  operation,  using  enamel  that  has  been  immersed  in 
1  per  cent,  hydrochloric  acid  for  a  number  of  hours.  Compare 
the  appearance  of  the  rods  with  those  of  the  former  specimen  and 
make  a  drawing  as  seen  with  the  high  power. 

Find  an  old  tooth  with  a  large  carious  cavity,  remove  the  softened 
dentin  without  touching  the  enamel  if  possible.  Lightly  scrape 
the  whitened  inner  surface  of  the  enamel  next  to  the  cavity  and 
mount  the  scrapings  as  before.  Compare  the  appearance  of  these 
rods  isolated  by  the  action  of  caries  with  those  of  the  previous 
specimen.  Notice  that  the  cross-markings  are  more  distinct  and 
the  expansions  and  constrictions  of  the  rods  more  prominent. 
Draw  a  few  of  the  rods  as  seen  with  the  high  power,  using  the  small 
diaphragm. 

PERIOD    V. 

Minute  Study  of  the  Enamel  and  Dentin. — Select  a  field  from 
one  of  the  ground  sections  where  the  specimen  is  very  thin,  and, 
if  possible,  where  the  entire  thickness  of  the  enamel  plate  can  be 
seen  in  one  field  with  the  f  objective.  To  select  this  field  all  of  the 
enamel  in  the  three  sections  should  be  carefully  studied  with  the 
low  power,  and  the  one  chosen  in  which  the  rods  can  be  seen  best 
and  can  be  most  easily  drawn.  Having  selected  the  field,  study 


388 


DIRECTIONS  FOR  LABORATORY   WORK 


the  enamel  with  the  high  power,  beginning  at  the  dento-enamel 
junction.  Note  the  form  of  the  dento-enamel  junction  and  the 
relation  of  the  two  tissues  at  this  point.  Note  the  diameter  of  the 


DENTO-ENAMEL   JCT 


FIG.  334. — High-power  drawing  of  the  enamel.     (Drawn  by  A.  B.  Hopper, 

1902-03.) 

enamel  rods  and  estimate  it,  using  a  red  blood  corpuscle  as  a  stand- 
ard of  measurement.  Note  the  striation  of  the  enamel  (page  67). 
Using  both  the  low  and  the  high  power,  draw  as  accurately  as 


MINUTE  STUDY  OF  CEMENTUM  AND  DENTIN         389 

possible  the  enamel  from  the  surface  to  the  dento-enamel  junction, 
showing  all  the  details  of  structure  that  can  be  made  out. 

The  drawing  should  be  made  as  long  as  the  page  will  allow,  and 
need  not  be  more  than  an  inch  wide,  and  should  include  just  enough 
of  the  dentin  to  show  the  dento-enamel  junction  and  the  character 
of  the  dentin  at  that  point  (Fig.  334).  Notice  the  diameter  of  the 
dentinal  tubules,  comparing  them  with  the  red  blood  corpuscles 
and  the  enamel  rods.  Note  the  amount  of  matrix  that  separates 
the  tubules.  Observe  the  forking  and  the  anastomosis  of  the 
tubules  as  they  approach  the  enamel,  and  follow  them  as  far  as 
possible. 

PERIOD   VI. 

Minute  Study  of  the  Cementum  and  Dentin. — With  the  low  power 
study  the  cementum  in  the  three  specimens,  looking  for  all  the 
details  of  structure  that  can  be  made  out  (see  page  154).  In  the 
gingival  portions  and  often  well  toward  the  apex,  especially  if  the 
tooth  is  from  a  young  person,  the  cementum  will  be  very  thin  and 
almost  structureless  in  appearance.  With  the  high  power,  fine 
lines  parallel  with  the  surface  may  be  seen,  which  indicate  the 
lamellae.  In  the  apical  portion  the  cementum  becomes  much 
thicker,  and  it  will  be  seen  that  each  layer  is  thicker  and  conse- 
quently more  easily  seen.  Little  black  spots  looking  like  spiders 
will  be  found  in  larger  or  smaller  numbers.  These  are  the  lacunae 
with  the  canaliculi  radiating  from  them.  They  were  filled  in  life 
by  cement  corpuscles.  Look  for  embedded  fibers  of  the  peridental 
membrane.  In  all  of  this  work  each  field  should  be  studied  with 
both  the  low  and  the  high  power. 

The  inner  layer  of  the  cementum  next  to  the  dentin  is  clear  and 
structureless,  and  the  dentin  adjoining  it  appears  with  the  low 
power  as  a  granular  layer  known  as  "the  granular  layer  of  Tomes." 
Studied  with  the  high  power,  the  appearance  will  be  seen  to  be 
caused  by  irregular  spaces  in  the  dentin  matrix  communicating 
with  the  dentinal  tubules  and  filled  in  life  with  protoplasm  of  the 
fibrils.  Compare  the  dentin  in  the  root  with  that  in  the  crown 
(page  143). 

After  studying  all  the  cementum  in  the  three  sections,  select 
three  fields,  one  from  the  gingival,  one  from  the  middle,  and  one 
from  the  apical  portion  of  the  root,  and  draw  the  tissues  from  the 
surface  of  the  root  to  the  pulp  chamber.  Show  all  the  details  of 
structure  that  can  be  made  out  with  both  low  and  high  powers 


390 


DIRECTIONS  FOR  LABORATORY  WORK 


(Fig.  335).     With  the  high  power  search  the  cementum  for  the 
record  of  absorptions  which  have  been  refilled  by  cementum. 


Mti>\ 


--CEMENTUM 


f DENTINE 


GRANULAR  LAYER 
OF   TOMES 


PULP  CHAMBER 


FIG.  335. — Cementum  and  dentin.     (Drawn  by  H.  J.  Lund  and  A.  E.  Hopper.) 

PERIOD   VII. 

Drawings  of  Typical  Cavity  Walls. — From  the  molar  or  bicuspid 
section  select  a  field  in  the  region  of  a  groove  or  pit.  Imagine  a 
cavity  to  be  prepared  in  this  position.  To  help  in  this,  an  ink 
line  may  be  made  on  the  cover-glass  by  using  a  fine  pen  and  India 


STUDY  OF  SECONDARY  DENTIN  AND  CEMENTUM     391 

ink,  or  ordinary  ink  to  which  a  little  sugar  has  been  added.  Now, 
using  both  the  high  and  the  low  power,  study  the  direction  of  the 
enamel  rods  as  they  appear  in  the  line  of  the  cavity  wall,  and  make 
a  drawing  showing  the  structural  requirements  for  a  good  wall  in 
this  position.  From  any  one  of  the  three  sections  select  a  field 
in  the  gingival  third  of  the  labial  or  buccal  surface  and  indicate 
the  line  of  a  cavity  wall  in  the  same  way.  Study  with  the  low  and 
the  high  powers  the  direction  of  the  enamel  rods  as  they  appear 
in  the  line  of  the  walls  of  the  cavity,  and  make  a  drawing  showing 
the  structural  requirements  for  good  walls  in  these  positions  (page 
107). 

PERIOD  vm. 

Outline  Drawings  from  Transverse  Sections  of  the  Root. — The 
ground  sections  of  the  root  have  been  prepared  and  should  be 
brought  to  the  laboratory  in  solution,  ready  for  mounting.  The 
three  sections  should  be  mounted  together  under  one  cover-glass, 
using  balsam  about  the  consistence  of  molasses.  The  sections  may 
be  studied  at  once,  but  after  the  day's  wrork  upon  them  they  should 
have  a  spring  clip  adjusted  to  the  cover-glass  and  be  put  aw^ay 
until  the  balsam  is  thoroughly  hard,  otherwise  they  may  work 
out  to  the  edge  of  the  cover-glass.  With  the  millimeter  gauge 
measure  the  length  and  breadth  of  each  section,  multiply  the 
measurements  by  twenty,  and  lay  off  a  rectangle  as  in  making  the 
longitudinal  drawings.  Draw  the  outline  of  the  section  and  the 
pulp  chamber  as  accurately  as  possible  before  studying  the  section 
with  the  microscope.  With  the  low  power  follow  the  dento- 
cemental  junction  around  each  section  and  draw  it  into  the  outline. 
Fill  in  half  of  each  section,  showing  the  direction  of  the  dentinal 
tubules,  the  position  and  character  of  the  granular  layer  of  Tomes, 
the  number  and  positions  of  the  lacunse,  and  the  other  structural 
characteristics  of  the  cementum.  In  this  study  the  record  of  the 
reduction  of  size  of  the  pulp  chamber  which  may  be  noted  by  changes 
in  the  direction  and  the  character  of  the  dentinal  tubules  (page 
152).  Label  the  section  with  the  name  of  the  root  from  which  it 
was  ground,  your  name,  and  the  date. 

PERIOD    IX. 

Study  of  Secondary  Dentin  and  Cementum. — With  the  low  power 
find  a  field  where  there  is  a  distinct  demarcation  between  dentin 


392  DIRECTIONS  FOR  LABORATORY  WORK 

of  earlier  and  later  formation,  and  draw  it  accurately  with  the 
high  power.  Compare  the  size  of  the  tubules,  their  number,  their 
direction,  and  their  diameter  in  the  earlier  with  the  later  formed 
dentin.  Is  there  any  connection  between  the  tubules  of  the  two 
portions?  Find  a  similar  field  from  a  longitudinal  section  and 
study  in  the  same  way,  making  an  accurate  drawing. 

Search  all  of  the  ground  sections  with  the  low  power  until  a  field 
is  found  where  the  dentinal  tubules  are  cut  transversely.  Adjust 
the  high-power  objective  and  study  the  field.  Notice  that  by 
focussing  up  and  down  with  the  fine  adjustment  the  tubules  seem 
to  move  in  a  circle,  showing  the  spiral  course  through  the  matrix. 
Using  a  red  blood  corpuscle  as  a  standard,  note  the  size  of  the 
subules,  their  distribution  in  the  matrix,  and  the  amount  of  matrix 
teparating  them.  Look  for  the  appearance  of  Newman's  sheath, 
which  is  that  portion  of  the  matrix  forming  the  immediate  wall 
of  the  tubule.  Draw  accurately  one  field  as  seen  with  the  high 
power.  Study  the  cementum  from  all  the  ground  sections  for  an 
area  showing  absorption  and  rebuilding,  and  if  found,  draw  one 
field  with  the  high  power.  Draw  five  or  six  lacunae  with  their 
canaliculi  as  seen  with  the  high  power,  selecting  as  great  a  variety 
of  forms  as  possible. 

PERIOD  X. 

Ground  Sections  of  Bone. — From  a  shaft  of  a  femur  or  humerus 
saw  a  disk  about  one-quarter  of  an  inch  thick.  In  doing  this 
notice  the  appearance  of  the  marrow  cavity  especially  as  you  look 
into  it  toward  the  articular  ends.  Saw  the  disk  into  sectors  with 
an  arc  of  about  a  quarter  of  an  inch  on  the  outer  surface.  From 
this  piece  saw  two  thin  slices— one  at  right  angles  to  the  axis  of 
the  bone,  the  other  parallel  with  it.  These  should  be  ground  as 
directed  in  the  introduction  for  the  grinding  of  transverse  sections 
of  the  root,  and  be  brought  to  the  laboratory  ready  to  mount. 
They  should  be  mounted  in  hard  balsam  as  described  in  the  mount- 
ing of  longitudinal  sections  of  the  teeth.  Label  the  slide  with  the 
name  of  the  bone  from  which  the  section  is  taken  and  the  direction 
in  which  it  is  cut.  Study  the  transverse  section  with  the  low 
power,  working  out  the  arrangement  of  the  lamellse  and  the  dis- 
tribution of  the  subperiosteal  and  Haversian  system  bone  (p. 
213).  Draw  the  tissue  from  the  surface  of  the  bone  to  the  marrow 
cavity.  This  drawing  should  be  not  more  than  an  inch  wide  and 


STUDY  OF  SUBPERIOSTEAL  BONE  AND  CEMENTUM     393 

the  full  length  of  the  page.  With  the  high-power  objective  draw 
one  or  two  Haversian  systems. 

Study  the  arrangement  of  the  Haversian  canals  as  seen  in  the 
longitudinal  sections.  With  the  high  power  draw  at  least  three 
lacunae,  showing  one  cut  lengthwise,  one  transversely,  and  one  as 
seen  from  above. 

PERIOD   XI. 

Decalcified  Bone. — One  of  the  bones  from  a  small  animal  has 
been  decalcified,  embedded,  sectioned,  and  stained  with  hematoxylin 
and  eosin.  Receive  from  the  desk  two  sections,  one  of  which  is 
cut  longitudinally,  the  other  transversely.  Mount  in  balsam  in 
the  usual  way.  Label  the  slide  with  the  name  of  the  animal,  the 
bone  from  which  it  is  cut,  and  the  direction  of  the  section.  Study 
the  transverse  section  with  the  low  power,  noting  the  bone  cor- 
puscles in  the  lacunae,  the  tissue  in  the  Haversian  canals,  and  the 
marrow.  With  the  high  power  draw  one  field  showing  two  or  three 
Haversian  systems,  one  of  which  has  been  partially  destroyed  in 
the  building  of  another.  Draw  with  the  high  power  one  field  from 
the  marrow  cavity.  From  the  longitudinal  section  draw,  with  the 
high  power,  one  field  showing  osteoblasts  in  a  medullary  space. 

PERIOD   XH. 

Comparative   Study  of   Subperiosteal  Bone   and  Cementum. — For 

this  day's  work  the  previously  mounted  sections  must  be  used, 
the  longitudinal  sections  of  the  teeth,  the  transverse  sections  of 
the  root,  the  ground  and  decalcified  sections  of  bone.  Study  the 
cementum  and  the  subperiosteal  bone  as  shown  in  these  sections 
and  make  one  drawing  of  cementum  and  one  drawing  of  sub- 
periosteal bone  to  show  the  comparison  in  structure.  Compare 
the  regularity  in  form  and  arrangement  of  the  lacunae  in  the  bone 
with  the  irregularity  in  form  and  position  of  the  lacunas  in  cemen- 
tum. Note  that  in  the  bone  the  lacunae  lie  between  the  layers; 
in  the  cementum  they  may  be  between  the  layers  or  entirely  within 
a  single  layer.  Compare  the  regularity  in  the  arrangement  and 
thickness  of  layers  with  the  corresponding  irregularity  in  cementum. 
Note  the  size,  number  and  arrangement  of  the  canaliculi  radiating 
from  the  lacunae  in  bone,  and  compare  them  with  the  canaliculi 
of  the  cementum. 


394  DIRECTIONS  FOR  LABORATORY  WORK 

PERIOD   XHI. 

Dental  Pulp  from  the  Unerupted  Tooth  of  a  Sheep. — An  unerupted 
molar  or  premolar  of  a  yearling  lamb  was  removed  from  the  lower 
jaw  by  splitting  the  bone.  The  pulp  was  pulled  out  of  the  partially- 
formed  dentin  embedded  in  paraffin,  sectioned,  stained  with  hema- 
toxylin  and  eosin.  Bring  to  the  desk  a  clean  slide  with  a  drop  of 
balsam  upon  the  center  of  it  and  receive  a  section.  Label  the 
slide:  "Pulp  from  unerupted  tooth  of  sheep,  stained  with  hema- 
toxylin  and  eosin."  Study  first  with  the  low  power.  Upon  the 
circumference  of  the  section  the  layer  of  odontoblasts  may  or  may 
not  be  shown,  depending  upon  whether  in  the  removal  of  the  pulp 
the  fibrils  have  pulled  away  from  the  dentin,  or  the  odontoblasts 
have  been  pulled  off  from  the  surface  of  the  pulp.  They  are  usually 
present,  at  least  in  spots.  Note  the  number  and  arrangement  of 
the  bloodvessels  and  the  distribution  of  the  connective-tissue 
cells.  With  the  low  power  draw  a  portion  from  the  surface  to  the 
center,  showing  the  layer  of  odontoblasts,  if  present.  With  the 
high  power  draw  one  field  showing  a  bloodvessel  and  the  connec- 
tive-tissue cells,  taking  particular  pains  to  represent  their  forms 
correctly.  If  there  are  any  odontoblasts  present  draw  one  field 
showing  them  and  the  layer  of  Weil  (see  page  167). 

PERIOD   XIV. 

Dental  Pulp,  Normal  Human. — A  number  of  human  teeth  were 
cracked  immediately  after  extraction  and  the  pulps  removed  from 
the  pulp  chambers.  They  were  embedded  in  one  block  of  paraffin, 
sectioned,  stained  with  hematoxylin  and  eosin,  and  are  ready  to 
be  given  out.  Bring  to  the  desk  a  clean  slide  with  a  drop  of  balsam 
on  the  center  and  receive  a  section.  Label  the  slide:  "Trans- 
verse section  of  pulp  from  human  teeth."  There  will  be  several 
sections  in  this  specimen,  each  from  a  separate  pulp.  With  the 
low  power  follow  the  circumference  of  each  section,  looking  for 
places  where  odontoblasts  are  present.  Find  the  best  field  in  the 
specimen  and  draw  the  layer  of  odontoblasts  as  seen  with  the 
high  power.  Notice  the  fibrils  which  have  been  pulled  out  of  the 
dentinal  tubules  projecting  from  the  ends  of  the  odontoblasts. 
If  the  section  is  parallel  with  the  long  axis  of  the  cells,  they  will 
appear  as  tall  columnar  cells  with  a  nucleus  in  the  deeper  end.  If 
it  is  oblique  to  their  axis  the  layer  may  appear  as  two  or  three 


ENDOCHONDRAL  BONE  FORMATION  395 

layers  of  oval  cells.  Just  beyond  the  odontoblasts  the  layer  of 
Weil  will  be  seen,  usually  appearing  as  a  clearer  layer  containing 
few  cells  and  about  half  as  wide  as  the  odontoblasts.  Beyond  this 
the  connective-tissue  cells  are  thickly  placed  for  a  short  distance, 
and  still  deeper  they  are  more  widely  scattered  and  about  evenly 
distributed  in  the  rest  of  the  pulp. 

With  the  high  power  draw  one  field  to  show  the  form  of  the 
connective-tissue  cells  of  the  pulp.  With  the  low  power  study  the 
distribution  of  the  bloodvessels  in  all  of  the  sections.  Select  the 
best  section  and  draw  the  entire  section  to  show  the  size,  number, 
and  arrangement  of  the  large  bloodvessels.  With  the  high  power 
draw  a  single  field  to  show  accurately  the  structure  of  a  bloodvessel 
wall. 

PERIOD   XV. 

Dental  Pulp,  Pathologic  Human. — By  the  cooperation  of  the  man 
in  charge  of  the  extracting  room,  or  an  extracting  specialist,  teeth 
with  living  but  inflamed  or  hyperemic  pulps  were  dropped  as 
soon  as  extracted  into  a  fixing  fluid.  The  teeth  wrere  afterward 
cracked  and  the  pulps  removed,  embedded,  and  sectioned  as 
before.  Bring  to  the  desk  a  clean  slide  with  a  drop  of  balsam  on 
its  center  and  receive  a  section.  Label  the  slide:  "Pathologic 
pulp  from  human  tooth  stained  with  hematoxylin  and  eosin." 
Follow  the  same  routine  in  studying  these  specimens  as  in  the 
case  of  the  normal  pulp.  It  is  impossible  to  tell  just  what  condi- 
tions will  be  present.  Compare  the  size  and  number  of  the  blood- 
vessels with  those  in  the  normal  tissue,  and  the  character  and 
distribution  of  the  cellular  elements.  Look  for  nodules  of  calco- 
globuli,  especially  in  the  inflammatory  specimens,  and  make  a 
diagnosis  of  the  condition,  as  showrn  in  the  specimen.  See  the 
chapter  on  the  Structural  Changes  in  the  Pulp  and  Pathological 
Conditions  for  further  assistance  on  the  work  in  this  material. 


PERIOD   XVI. 

Endochondral  Bone  Formation. — A  forming  bone  from  a  human 
fetus  has  been  embedded,  sectioned,  and  stained  with  hematoxylin 
and  eosin.  Receive  a  section  from  the  desk  and  mount  as  usual. 
Study  the  specimen  with  the  low  power,  identifying  first  the  gen- 
eral arrangement  of  the  tissues,  following  from  the  unchanged 


396  DIRECTIONS  FOR  LABORATORY  WORK 

cartilage  to  the  development  of  bone.  Notice  the  subperiosteal 
layers  on  the  surface.  Make  a  sketch  of  a  sufficient  part  of  the 
section  to  show  the  changes  from  the  typical  hyaline  cartilage  to 
the  young  bone.  With  the  high  power  draw  one  field  from  a  primary 
marrow  cavity,  showing  osteoblasts  laying  down  lamellae  on  one 
of  the  spicules,  and  one  field  showing  osteoclasts. 


PERIOD   XVH. 

Bone  Growth. — A  piece  of  a  long  bone  from  a  very  young  animal 
has  been  embedded  and  sectioned  transversely  to  the  shaft.  Sec- 
tions have  been  stained  in  hematoxylin  and  eosin,  to  be  mounted 
as  usual.  Label  the  slide:  "Growing  bone  cut  transversely, 
stained  with  hematoxylin  and  eosin."  Study  first  with  the  low 
power.  On  the  surface  of  the  section  will  be  seen  the  periosteum, 
in  which  the  fibrous  and  osteogenetic  layers  can  be  easily  recog- 
nized. Bone  formation  is  actively  going  on,  laying  down  lamellae 
under  the  periosteum  which  are  being  transformed  into  Haversian 
system  bone.  With  the  low  power  draw  a  portion  of  the  section 
from  the  periosteum  to  the  center  of  the  bone.  With  the  high 
power  draw  a  field  showing  the  osteoblasts  of  the  periosteum,  a 
field  showing  the  absorption  of  subperiosteal  bone  to  form  a  medul- 
lary space,  and  a  field  showing  osteoblasts  in  a  medullary  space. 


PERIOD  xvm. 

Periosteum  from  Attached  Portion. — From  a  young  kitten  a  por- 
tion of  a  bone  in  a  region  to  which  muscles  are  attached  to  the 
periosteum  was  carefully  dissected  out,  removing  the  attached 
muscle,  and  the  tissue  embedded  in  celloidin,  the  sections  cut 
parallel  to  the  axis  of  the  bone  and  perpendicular  to  its  surface. 
They  have  been  stained  in  hematoxylin  and  eosin,  and  are  ready 
to  be  given  out.  Receive  a  section  and  mount  as  usual.  Label: 
"Periosteum  from  attached  portion,  stained  in  hematoxylin  and 
eosin."  Study  the  specimen  first  with  the  low  power.  The  outer 
fibrous  layer  of  the  periosteum  will  be  seen  with  the  muscle  fibers 
attached  to  it  and  the  osteogenetic  layer  with  the  greater  number 
of  cells  taking  the  stain  more  deeply.  Draw  with  the  low  power, 
showing  the  tissues  from  the  surface  of  the  periosteum  well  into 
the  substance  of  the  bone.  With  the  high  power  study  the  attach- 


GINGIVUS  AND  GUM   TISSUE  397 

ment  of  the  muscle  fibers  to  the  outer  layer  of  the  periosteum, 
the  character  and  arrangement  of  the  fibers  of  the  outer  layer, 
the  interlacing  of  the  fibers  of  the  outer  and  inner  layer,  the  cells, 
and  especially  the  osteoblasts  of  the  inner  layer  and  the  penetrating 
fibers  that  are  built  into  the  bone.  Draw  the  thickness  of  the 
periosteum  as  seen  with  the  high  power,  showing  the  details  of 
structure  as  accurately  as  possible. 


PERIOD   XIX. 

Gingivus  and  Gum  Tissue. — The  gingivus  and  gum  tissue  covering 
the  alveolar  process  down  to  the  point  of  reflection  on  to  the  cheek 
was  dissected  away  from  the  teeth  and  jaw  of  a  sheep.  The  tissue 
was  embedded  in  paraffin  and  sectioned  parallel  with  the  long  axis 
of  the  tooth.  The  sections  have  been  stained  with  hematoxylin 
and  Van  Gieson,  and  are  ready  to  mount.  Bring  to  the  desk  a 
clean  slide  with  a  drop  of  balsam  on  the  center  and  receive  a  speci- 
men. Label  the  section:  "Gingivus  from  a  sheep,  stained  with 
hematoxylin  and  Van  Gieson."  By  this  staining  the  cellular 
elements  will  have  a  brownish  color,  the  nuclei  dark,  the  protoplasm 
lighter,  the  white  fibers  should  be  bright  red,  and  the  elastic  fibers 
yellowish.  It  is  a  specially  good  stain  for  connective  tissue.  Study 
with  the  low  power.  The  epithelium  will  be  stained  a  brownish 
yellow  or  purple.  It  is  a  stratified  squamous  epithelium  made  up 
of  many  layers  of  cells  and  with  a  distinct  horny  or  corneous  layer 
on  the  surface  from  the  crest  of  the  gingivus  to  the  point  where 
the  mucous  membrane  is  reflected  on  to  the  cheek,  or  where  it 
ceases  to  be  attached  to  the  gum.  This  layer  is  yellowish  in  color, 
and  is  made  up  of  closely  packed  scales  having  no  nuclei.  They 
are  the  remains  of  epithelial  cells  from  which  the  protoplasm  is 
gone,  leaving  only  the  horny  material  which  it  had  produced.  The 
portion  of  the  epithelial  lining  the  gingival  space  has  no  corneous 
layer,  nuclei  being  seen  in  the  cells  at  the  surface.  The  cells  are 
larger  and  more  loosely  placed.  The  connective-tissue  papillae 
and  the  projections  of  epithelium  which  are  between  them  are 
extremely  long.  In  the  epithelium  covering  the  alveolar  process 
the  connective-tissue  papillae  are  broader  and  not  so  deep,  and  the 
cells  are  much  more  compactly  arranged.  At  the  point  of  reflec- 
tion on  the  cheek  the  epithelium  changes  its  character  abruptly, 
the  corneous  layer  disappears,  the  surface  cells  showing  nuclei, 


398  DIRECTIONS  FOR  LABORATORY  WORK 

the  epithelial  layer  is  thicker  and  made  up  of  larger  and  more 
loosely  placed  cells.  This  change  in  the  structure  explains  why 
the  epithelium  is  easily  broken  where  a  movable  portion  of  the 
membrane  passes  over  the  edge  of  an  artificial  denture.  When 
an  infection  reaches  the  connective  tissue  a  sore  is  produced  that 
requires  some  time  to  heal. 

Study  the  connective  tissue,  which  is  made  up  of  coarse,  wavy 
bundles  of  white  fibers  taking  the  red  stain.  In  the  gum  tissue, 
that  is,  the  portion  of  the  section  covering  the  alveolar  process, 
the  bundles  are  very  large  and  form  a  very  coarse  network.  Beyond 
the  point  of  reflection  the  bundles  are  finer  and  more  delicate 
in  their  arrangement.  Elastic  fibers  take  the  yellowish  stain. 
Notice  the  bloodvessels  in  the  connective  tissue  and  the  capillaries 
in  the  papillae.  With  the  low  power  draw  the  entire  section  so  as 
to  show  the  character  of  the  epithelium  and  the  fibrous  tissue  in 
the  three  parts. 

With  the  high  power  draw  the  thickness  of  the  epithelium  lining 
the  gingival  space  and  at  the  point  where  the  membrane  is  reflected 
to  the  cheek. 

PERIOD   XX. 

Peridental  Membrane,  Transverse  Gingival. — The  lower  jaw  of  a 
young  sheep  was  sawed  through  between  the  teeth,  cutting  the 
jaw  into  blocks  each  containing  two  teeth.  The  crowns  were  broken 
off  or  opened  so  as  to  admit  the  fluids  to  the  pulp  tissue.  The  tissues 
were  decalcified,  embedded,  and  sectioned  at  right  angles  to  the 
axis  of  the  tooth.  They  are  cut  from  the  gingival  portion,  and  have 
been  stained  with  hematoxylin  and  eosin.  Receive  a  section  and 
mount  as  usual.  Label  the  slide:  "Peridental  membrane,  trans- 
verse gingival,  stained  with  hematoxylin  and  eosin."  A  similar 
block  of  tissue  preserved  in  alcohol  will  be  found  at  the  desk.  This 
should  be  observed  so  as  to  study  out  the  relation  of  the  section 
to  the  gross  appearance  of  the  tissue. 

Holding  the  section  to  the  light,  observe  the  distribution  of  the 
tissue.  Two  roots  will  be  seen  cut  across.  Observe  the  epithelium 
on  the  labial  and  the  lingual,  and  possibly  also  that  lining  the 
gingival  space  lying  next  to  the  root  of  one  of  the  teeth.  By  the 
aid  of  the  low  power  sketch  the  outline  of  the  entire  section  to 
show  the  distribution  of  the  tissues.  Note  the  demarcation  where 
the  finer  fibers  of  the  peridental  membrane  unite  with  the  coarser 


PERIDENTAL  MEMBRANE  399 

mat  of  gum  tissue.  Beginning  at  the  center  of  the  labial  surface, 
follow  the  fibers  springing  from  the  cementum  to  where  they  are 
lost  in  the  gum  tissue  or  attached  to  the  approximating  tooth. 
Draw  the  portion  of  the  membrane  between  the  two  roots,  accu- 
rately representing  the  arrangement  of  the  fibers.  The  epithelial 
structures  will  be  seen  lying  between  the  fibers  close  to  the  cemen- 
tum, and  should  be  shown  in  the  drawing  (p.  248). 

With  the  high  power  study  the  cementoblasts  and  the  epithelial 
structures.  Make  a  drawing  of  one  field,  showing  all  the  details 
of  structure  as  accurately  as  possible. 

With  the  high  power  draw  one  field  showing  the  fibrous  tissue 
between  the  roots  and  the  relation  of  the  fibroblasts  to  them. 
This  field  should  include  a  bloodvessel. 


PERIOD   XXI. 

Peridental  Membrane,  Alveolar  Portion,  Transverse. — The  sections 
for  this  work  have  been  cut  from  the  same  block  as  the  preceding, 
but  are  in  the  occlusal  third  of  the  alveolar  portion  and  as  close 
to  the  border  of  the  alveolar  process  as  possible.  Receive  a  section. 
Mount  as  usual  and  label  the  slide:  "Peridental  membrane, 
alveolar  portion,  transverse,  stained  with  hematoxylin  and  eosin." 

Study  the  general  arrangement  of  the  tissues  and  make  a  sketch 
as  in  the  case  of  the  previous  specimen.  Note  the  muscle  fibers 
from  the  muscles  of  the  lip  attached  to  the  periosteum  on  the 
labial  surface  of  the  process,  the  bone  of  the  labial  plate,  the  sep- 
tum separating  the  alveoli,  the  peridental  membrane  filling  the 
space  between  the  bone  and  the  surface  of  the  root,  the  layers  of 
the  cementum,  the  dentin  and  the  pulp. 

After  studying  the  specimen  with  the  low  power  as  carefully  as 
possible,  draw  the  peridental  membrane  surrounding  one  root, 
including  the  thickness  of  the  labial  plate  of  bone  with  its  perios- 
teum and  a  part  of  the  lingual  plate.  In  this  drawing  represent 
accurately  the  fibers  of  the  peridental  membrane,  their  arrangement 
in  the  bundles,  and  the  relation  of  the  bundles  to  each  other  and 
the  bloodvessels.  To  do  this  the  fine  adjustment  must  be  used  to 
obtain  ideas  of  the  third  dimension  of  space.  With  the  high  power 
draw  one  field  from  the  wall  of  the  alveolus,  showing  the  attach- 
ment of  the  fibers  to  the  bone,  the  osteoblasts  on  the  surface  of 
the  bone,  and  the  other  cellular  elements.  This  field  should  include 


400  DIRECTIONS  FOR  LABORATORY  WORK 

a  bloodvessel.  With  the  high  power  draw  the  thickness  of  the 
cementum  at  some  point  where  a  specially  strong  bundle  of  fibers 
is  attached.  This  should  show  the  fibers  embedded  in  the  cemen- 
tum, cementoblasts  on  the  surface,  and  the  branching  and  inter- 
lacing of  the  bundles. 

PERIOD   XXII. 

Longitudinal  Section  of  the  Peridental  Membrane. — The  lower 
incisor  of  a  young  sheep  was  removed  from  the  jaw  by  sawing 
through  between  the  teeth,  leaving  two  teeth  in  each  block.  The 
crowrns  of  the  teeth  were  broken  off  near  the  level  of  the  gum  so 
as  to  admit  the  reagents  to  the  pulp  chamber.  The  tissues  decalci- 
fied, embedded  in  celloidin,  and  sectioned.  They  were  cut  through 
from  labial  to  lingual,  and  only  the  ones  from  the  central  portion 
used.  They  have  been  stained  in  hematoxylin  and  eosin  and  are 
ready  to  mount.  Mount  the  section  as  usual  and  label  the  slide: 
"Longitudinal  section  through  the  peridental  membrane  of  a 
sheep,  labiolingual,  stained  in  hematoxylin  and  eosin."  First  hold 
the  section  up  to  the  light  and  note  the  relation  of  the  tooth  to  the 
bone  and  the  soft  tissues.  Study  the  section  with  the  low  power 
and  make  a  sketch  showing  the  general  distribution  of  the  tissues. 
Show  the  pulp  chamber,  dentin  and  cementum,  bone,  periosteum, 
gum  tissue,  and  epithelium.  Do  not  attempt  to  fill  in  the  drawing 
more  than  diagrammatically,  for  it  would  require  too  much  time. 
The  object  of  the  drawing  is  to  get  the  general  relation  of  the 
tissue  before  studying  parts  of  it  in  detail.  Compare  the  form  of 
the  labial  and  the  lingual  gingivus  and  make  a  drawing  of  the 
lingual,  showing  the  details  of  structure  as  far  as  the  border  of  the 
process  and  as  accurately  as  possible.  With  the  high  power  draw 
the  thickness  of  the  epithelium  lining  the  gingival  space.  Study 
the  fibers  in  the  occlusal  third  of  the  alveolar  process  and  make  a 
drawing  to  represent  them  accurately,  showing  the  cementum 
at  one  side  and  the  bone  at  the  other.  The  entire  length  of  the 
root  can  seldom  be  got  in  one  section  on  account  of  the  curve  of 
the  tooth,  so  that  the  fibers  can  probably  be  studied  to  advantage 
in  the  occlusal  third  of  the  alveolar  process  only.  Draw  one  field 
with  the  high  power  showing  the  bloodvessels. 


TOOTH  GERM  401 

PERIOD   XXIII. 

Tooth  Germ. — The  head  of  an  embryo  pig  was  embedded  in 
paraffin  and  sectioned  at  right  angles  to  the  snout.  The  sections 
begin  in  the  region  of  the  incisors  and  far  enough  back  to  cut  through 
the  nose  cavity.  They  have  been  stained  in  hematoxylin  and  eosin. 
Bring  to  the  desk  a  clean  slide  and  receive  a  section.  Label  the 
slide:  "Tooth  germ,  stained  with  hematoxylin  and  eosin." 

The  general  form  of  the  section  will  depend  on  the  position  of 
the  section  through  the  head.  At  the  desk  is  the  head  of  a  similar 
embryo  preserved  in  alcohol.  This  should  be  observed  so  as  to 
determine  from  the  section  its  relation  to  the  head.  By  holding 
the  section  to  the  light  and  the  use  of  the  low  power,  make  a  sketch 
of  the  entire  section.  Note  the  epiblast  covering  the  outer  surface 
and  lining  the  nose  and  mouth  cavity.  The  mass  which  is  to  form 
the  tongue  lying  between  the  roof  of  the  mouth  and  the  mandibular 
arch.  If  the  section  is  in  front  of  the  angle  of  the  mouth  there  will 
be  no  connection  between  the  upper  and  lower  parts  of  the  section. 
Notice  the  separation  of  the  nose  cavity  into  right  and  left  by  a 
septum  containing  cartilage,  and  the  projections  of  cartilage  from 
the  side  walls  which  will  form  the  turbinate  bones.  On  either  side 
of  the  septum  where  it  joins  the  palate  will  be  seen  little  structures 
known  as  Jacobson's  organ,  which  later  disappear.  Notice  Meckel's 
cartilage  in  the  mesodermic  mass  of  the  mandible.  In  the  epiderm 
of  the  outer  surface  the  beginning  of  the  formation  of  hairs  are  to 
be  seen. 

^Yith  the  low  power  follow  the  epiderm  lining  the  mouth  cavity 
and  look  for  the  tooth  germ.  In  each  section  there  are  four  chances 
for  tooth  germs,  one  on  either  side  in  the  upper  and  lower  arches. 
Select  the  best  one  and  draw  it  as  seen  with  the  low  power.  The 
appearance  will  depend  entirely  upon  the  stage  of  development. 

\Yith  the  high  power  draw  enough  of  the  enamel  organ  to  show 
the  arrangement  of  the  cells  in  the  outer  and  inner  tunics  and  the 
stellate  reticulum. 

PERIOD   XXIV. 

Tooth  Germ. — Sections  have  been  prepared  in  the  same  way  as 
in  the  preceding,  but  from  the  head  of  an  older  embryo,  in  which 
the  tooth  germs  are  completely  formed  and  calcification  is  ready 
to  begin. 
26 


402  DIRECTIONS  FOR  LABORATORY  WORK 

Receive  a  section,  mount,  and  label  as  before,  and  draw  the 
outline  of  the  entire  section.  Note  the  changes  in  form  and  in 
the  tissue  elements  from  the  previous  section.  Bone  formation 
has  begun  both  in  the  mandible  and  the  maxilla.  The  amount  and 
distribution  of  this  should  be  carefully  studied. 

With  the  low  power  draw  the  entire  tooth  germ,  selecting  the 
most  typical  one  in  the  section.  With  the  high  power  draw  one 
field  showing  ameloblasts,  odontoblasts,  and  a  portion  of  the 
papillae.  Find  a  field  in  which  bone  formation  is  going  on  and 
draw  it  accurately  with  the  high  power. 


APPENDIX  CHAPTER  I. 


THE   GRINDING   OF   MICROSCOPIC   SPECIMENS,    USING 
THE   GRINDING   MACHINE. 

BY  G.  V.  BLACK,  M.D.,  D.D.S.,  Sc.D.,  LL.D. 

The  Machine. — The  basis  of  this  machine  is  the  larger  watch- 
maker's lathe  known  as  No.  2.  It  must  swing  4  inches,  the  length 
of  the  bed  must  be  12  inches,  and  be  good  and  solid.  A  test  should 
be  made  of  the  alignment  of  the  lathe  head  to  see  that  this  is  exact. 
If  there  is  any  inaccuracy,  another  lathe  should  be  selected.  The 
power  should  consist  of  one  of  the  largest  and  strongest  electric 
lathes,  or  motors,  made  for  the  use  of  dentists.  This  power  should 
be  transmitted  to  the  lathe  through  an  overhead  shaft  of  a  length 
that  will  give  good  room  to  operate  the  lathe  without  the  motor 
being  in  the  way.  A  pulley  may  be  placed  on  the  left  end  of  the 
shaft  of  the  motor  on  one  of  the  brass  carriers  for  grinding  wheels. 
This  pulley  should  carry  a  good  quarter-inch  round  leather  belt. 
Its  diameter  should  be  2^  inches.  The  pulley  on  the  right  hand 
end  of  the  shaft  above  should  be  5  inches.  This  will  reduce  the 
speed  one-half  and  double  the  power.  On  the  left  end  of  the  shaft 
should  be  placed  a  copy — reversed — of  the  pulley  on  the  lathe- 
head,  which  has  4  grooves.  This  gives  good  varieties  of  speed 
with  each  speed  of  the  motor.  Another  small  pulley  will  be  placed 
near  the  center  of  the  length  of  the  overhead  shaft,  the  purpose 
of  which  will  be  explained  later  (Figs.  336  and  337). 

The  grinding  apparatus  is  built  upon  a  base  fitted  to  the  lathe 
bed  in  the  same  way  as  the  lathe  head,  or  tail-piece.  It  has  one 
main  shaft  parallel  with  the  lathe  bed,  in  good  and  sufficient  bear- 
ings to  maintain  accuracy  of  alignment  and  perfect  steadiness 
for  long-continued  usage  (see  Figs.  336  and  337).  This  shaft  moves 
freely  lengthwise,  or  back  and  forward,  while  turning  slowly  in 
its  bearings.  On  the  end  of  this  shaft  next  to  the  lathe  head — the 
forward  end — there  is  a  larger  portion,  or  ring,  and  this  end  ter- 

(403) 


404 


APPENDIX  CHAPTER  I 


minates  in  a  threaded  nipple,  upon  which  the  removable  grinding 
disks  are  screwed  firmly  against  the  face  of  this  larger  ring,  to 


FIQ.  336 


FIGS.  336  and  337.- — -A  general  view  of  the  grinding  machine,  showing  particularly 
the  arrangement  for  transmitting  the  power  from  the  electric  motor  to  the  machine 
that  does  the  work.  All  of  this  may  be  made  out  by  reference  to  the  picture  while 
following  the  text.  The  bed  of  the  little  lathe  on  the  left  hand  is  12j  inches  long, 
which  gives  a  good  idea  of  the  general  dimensions. 

The  water  is  delivered  to  the  grinding  stone  from  a  rubber  bag  or  bucket  hung  on 
the  frame  above  through  a  rubber  tube  to  the  metal  tube  on  a  movable  stand,  which 
may  be  so  placed  as  to  bring  the  brush  at  its  end  against  the  stone.  This  stand  and 
brush  are  better  seen  in  Fig.  337. 


secure  accuracy  of  adjustment, 
more  fully  explained  later. 


The  use  of  these  disks  will  be 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS 


405 


On  the  rear  end  of  this  shaft,  just  back  of  its  rear  bearing  and 
abutting  against  it,  a  large  movable  nut  is  placed.  This  is  pro- 
vided with  a  thumb  screw  by  which  it  is  made  fast  at  any  point 
desired.  Turning  this  forward  pulls  the  shaft  back  from  the  grind- 
ing stone.  Turning  it  backward  allows  the  shaft  to  move  forward 

FIG.  337 


against  the  stone.  It  has  also  a  finger  reaching  back  over  a  grad- 
uated disk  just  to  its  rear.  This  disk  is  made  fast  on  the  shaft, 
and  the  two  together  constitute  the  micrometer,  by  which  the 
thickness  to  which  specimens  are  ground  is  measured.  The  movable 
nut  has  40  threads  to  the  inch.  The  graduation  of  the  disk  is  on 
the  same  principle  as  that  on  the  screw  calipers  used  by  machinists 


406 


APPENDIX  CHAPTER  1 


for  fine  measurements — one-thousandth  of  an  inch — but  as  this 
disk  is  If  inches  in  diameter,  the  graduations  of  thousandths  are  so 


FIG.  338 


FIGS.  338  and  339.— The  lathe  with  the  grinding  machine  mounted  upon  it  in  position 
for  work.  On  the  left  next  to  the  lathe  head  is  the  grinding  stone  surrounded  by  the 
spatter  guard,  which  gathers  all  of  the  water  from  the  wheel  and  delivers  it  through 
its  hollow  post  into  a  rubber  tube  below  the  lathe  bed,  which  conveys  it  to  a  con- 
veniently placed  receptacle.  The  water  comes  from  a  rubber  bag  or  bucket  hung 
on  the  overhead  frame  (see  Fig.  336)  through  a  rubber  tube  to  the  metal  tube  mounted 
on  a  movable  stand  so  that  the  brush  through  which  it  passes  may  be  placed  against 
the  stone.  The  grinding  machine  proper  is  secured  to  the  lathe  bed  by  the  larger 
thumb-screw  seen  below.  The  point-finder  is  seen  at  the  foot  of  the  spatter  guard, 
and  is  secured  by  the  middle  thumb-screw  seen  below  the  lathe  bed. 

The  shaft  of  the  grinding  machine  (6  inches  long)  runs  through  its  whole  length, 
but  is  completely  covered  in  by  its  housings  to  protect  its  bearings  from  grit,  except 
at  its  forward  end  (next  to  the  grinding  stone).  This  part  is  protected  by  a  swaddle 
held  by  a  ring,  which  keeps  the  working  bearing  clean.  On  this  end  the  grinding 
disk  is  seen  almost  touching  the  stone.  The  micrometer  is  on  the  other  end  of  the 
shaft  next  back  of  the  frame  of  the  grinding  machine.  Next  back  of  this  is  a  toothed 
wheel  made  fast  to  the  shaft.  This  is  actuated  by  the  middle  one  of  the  belts  descend- 
ing from  overhead  (Fig.  336,  the  left  hand  belt  in  Fig.  337).  This  belt  passes  over 
a  wheel  hidden  from  view  and  through  a  small  worm  shaft  turns  the  main  shaft. 
Pressure  for  the  grinding  is  supplied  by  a  plunger  actuated  by  a  spiral  spring -seen 
at  the  extreme  right-hand  end. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS 


407 


wide  that  one-quarter  of  one-thousandth  may  readily  be  used.    It 
differs  in  plan,  in  that  both  the  graduation  and  the  parallel  lines 


are  placed  upon  this  disk.  On  the  machinist's  micrometer  the 
lines  are  placed  on  the  shaft  and  the  graduations  on  the  nut.  The 
graduation  is  read  from  the  side  of  the  finger  on  the  movable  nut, 


408  APPENDIX  CHAPTER  I 

and  the  lines  are  read  from  its  end.    It  is  a  very  perfect  micrometer 
(Figs.  338  and  339). 

The  forward  movement  of  the  shaft  when  grinding,  and  also 
the  pressure  exerted  upon  the  stone,  are  furnished  by  a  tail-piece 
placed  behind  it  and  attached  to  the  lathe  bed.  This  has  a  plunger 
actuated  by  a  spiral  spring,  which  pushes  the  shaft  forward  against 
the  stone.  The  amount  of  pressure  exerted  in  the  grinding  is  con- 
trolled by  the  amount  of  compression  of  this  spring  in  fixing  the 
piece  to  the  lathe  bed.  It  may  be  much  or  little,  as  desired.  Usually 
very  little  pressure  is  used.  When  the  movable  nut  has  come 
against  the  frame  in  which  this  shaft  turns,  the  machine  may 
continue  to  run,  but  the  forward  movement  of  the  shaft  stops  and 
the  grinding  ceases  in  consequence.  Therefore  there  is  no  danger 
of  grinding  a  specimen  thinner  than  the  measurement  fixed  upon. 
The  further  arrangement  for  finding  this  measurement  will  be 
described  later. 

On  the  rear  portion  of  the  graduated  disk,  or  wheel,  a  portion 
or  space  is  toothed,  and  connected  with  a  worm  pinion  or  threaded 
shaft  by  which  the  main  shaft  is  turned  in  its  bearings.  A  belt  is 
attached  over  a  wheel  on  the  end  of  this  worm  shaft,  and  extends 
to  the  third  wheel,  previously  mentioned,  on  the  overhead  shaft. 
When  this  belt  is  adjusted  and  the  motor  started,  it  causes  the  main 
shaft  in  the  grinding  machine  proper  to  turn  slowly  on  its  axis, 
while  being  pressed  against  the  stone  by  the  tail-piece.  By  this 
arrangement  every  part  of  the  specimen  fixed  on  the'  grinding  disk 
is  brought  successively  against  every  part  of  the  rapidly  revolving 
stone,  and  is  cut  perfectly  level  in  all  of  its  parts. 

The  Grinding  Disks. — The  grinding  disks  are  of  brass,  accurately 
turned  f  inch  thick,  and  If  inches  in  diameter.  They  have  a 
threaded  hole  |  inch  deep  in  the  back  to  fix  them  to  the  nipple  on 
the  forward  end  of  the  shaft  of  the  grinding  machine.  A  machine 
should  have  a  half-dozen  or  more  of  these,  lettered  or  numbered 
on  the  edge,  so  that  records  of  each  may  be  made  when  measuring 
preparatory  to  mounting  specimens  for  grinding.  As  the  mounting 
of  specimens  on  others  of  these  may  proceed  while  the  grinding 
on  one  is  going  on  (for  the  machine,  being  automatic,  needs  little 
attention),  this  number  at  the  least  is  necessary  for  rapid  work. 

The  machine  may  be  stopped  and  the  disk  removed  from  the 
shaft  by  a  few  backward  turns,  the  progress  of  the  grinding  exam- 
ined, the  disk  returned  for  further  grinding,  etc.,  at  any  time 
during  the  progress  of  the  work.  The  face  of  the  disk,  which 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          409 

should  be  perfectly  flat  and  parallel  with  the  face  of  the  stone, 
should  always  be  perfectly  bright,  so  as  to  reflect  light  through 
the  specimen  when  it  becomes  thin.  This  enables  one  to  judge 
very  closely  of  the  thickness  by  the  eye  (after  sufficient  practice), 
that  sometimes  proves  a  valuable  check  on  the  setting  of  the 
measurement  in  the  beginning. 

The  Point-finder.— This  is  a  piece  of  steel  one-eighth  of  an  inch 
thick,  fitted  to  the  lathe  bed  and  set  against  the  face  of  the  lathe 
head,  and  made  fast  by  a  thumb-screw  passing  through  the  lathe 
bed  from  below.  It  has  a  strong  arm  which  passes  around  other 
fixtures  between  the  lathe  head  and  the  forward  end  of  the  base 
of  the  grinding  machine.  It  is  provided  with  a  set-screw,  by  which 
a  range  of  variation  can  be  made  in  the  distance  of  the  forward 
end  of  the  frame  of  the  grinding  machine  from  the  lathe  head. 
When  this  is  in  place  and  the  measurement  of  a  disk  has  been  made 
and  recorded  for  the  grinding  of  a  specimen  to  a  specified  thickness, 
the  machine  may  be  taken  to  pieces  and  set  up  again  and  the 
grinding  proceed  without  fear  of  disturbing  the  measurement,  so 
long  as  the  set-screw  in  the  point-finder  is  not  moved.  It  is  often 
necessary  during  grinding  to  loosen  the  grinding  machine  from  the 
lathe  bed,  slide  it  back  to  adjust  something,  to  remove  disks  for 
examination  of  the  progress  of  the  work,  etc.  This  point-finder, 
by  preserving  the  distance  between  the  lathe  head  and  the  grinding 
machine,  enables  one  to  do  this  at  will,  and  again  find  his  exact 
point  of  measurement  simply  by  sliding  the  frame  of  the  grinding 
machine  forward  against  the  set-screw  of  the  point-finder.  This 
little  device  seems  absolutely  necessary  to  the  highest  usefulness 
of  the  machine. 

Lap  Wheels  and  Grinding  Stones. — I  began  my  work  of  grinding 
specimens  by  the  use  of  lap  wheels,  but  soon  discarded  them  because 
they  were  dirty.  They  cut  much  quicker  than  stones,  however, 
and  may  be  used  for  the  bulk  of  the  work  when  much  grinding  of 
very  hard  material  is  to  be  done.  They  are  not  necessary  in  grind- 
ing teeth,  bone,  etc.,  but  in  grinding  the  harder  fossils,  especially 
those  impregnated  with  the  silica,  and  in  some  geological  work 
they  become  necessary. 

The  best  lap  wheel  I  have  used  is  an  aluminum  wheel.  Brass 
or  iron  will  do  the  work,  but  aluminum  holds  the  grit  better,  cuts 
with  lighter  pressure,  and  does  the  work  quicker.  In  using  these 
I  have  fed  them  continuously  by  hand  with  carborundum  powder 
in  soapy  water,  using  a  brush. 


410  APPENDIX  CHAPTER  I 

The  Stones. — Anyone  who  is  doing  much  grinding  should  have 
a  good  supply  of  stones.  I  have  a  pair  of  carborundum  wheels, 
a  pair  of  emery  wheels,  a  pair  of  India  oil  stones,  and  a  pair  of 
Arkansas  stones.  In  each  of  these  pairs  one  is  fine  and  the  other 
coarser  grit.  Every  stone  is  dressed  to  a  perfect  face  on  the  lathe 
head  where  it  is  to  do  its  work,  with  a  black  diamond  held  in  the 
slide  rest. 

These  stones,  when  put  in  good  shape,  seem  capable  of  doing  an 
unlimited  amount  of  work.  The  conditions  of  the  grinding  pre- 
vents them  from  getting  out  of  true.  All  that  seems  necessary  is 
to  roughen  them  a  bit  with  a  picking  wheel  when  they  become 
too  smooth  to  cut  well.  For  this  purpose  a  much  smaller  picking 
tool  than  the  smallest  sold  for  the  general  mechanical  uses  seems 
desirable.  This  picking  wheel  has  sharp  teeth  of  the  hardest  steel 
possible  on  its  periphery.  It  is  held  in  a  handle  in  such  form  that 
the  wheel  is  free  to  turn.  In  use  it  is  held  against  the  rapidly 
rotating  stone  and  slowly  passed  over  its  entire  surface.  It  may 
be  held  in  the  hand  aided  by  a  tool  rest,  or  may  be  arranged  for 
use  in  the  slide  rest,  which  is  the  better  form  for  this  work. 

Watering  the  Stones. — In  grinding,  the  stones  are  kept  wet  in 
running  ice  water.  A  balsam  that  is  too  soft  to  hold  a  specimen  for 
grinding  in  water  at  room  temperature  will  hold  it  perfectly  in 
ice  water,  because  it  is  much  harder  when  cold.  For  this  purpose, 
a  receptacle  for  ice  is  hung  on  the  frame  that  holds  the  overhead 
shaft,  and  filled  with  bits  of  ice  and  then  filled  with  water.  Both 
the  ice  and  the  water  must  be  clean,  for  the  opening  in  the  tube 
where  it  passes  the  valve  which  regulates  the  flow  is  very  small, 
and  a  little  bit  of  dirt  or  trash  might  stop  the  flow.  In  this  case 
the  specimen  being  ground  would  be  burned  instantly.  A  bucket, 
or  a  large  rubber  bag,  will  answer  for  this  purpose.  Then  an  ordi- 
nary rubber  tube  answers  to  conduct  the  water.  It  is  best  to  have 
this  rubber  tube  to  connect  with  a  metal  tube  mounted  on  a  stand 
that  may  be  placed  in  any  position  wanted  to  deliver  the  water 
to  the  stone.  This  metallic  tube  is  provided  with  a  valve  for  the 
regulation  of  the  flow.  In  its  final  end  it  should  be  provided  with 
a  brush  of  rather  long  bristles,  into  which  the  water  is  delivered 
and  spread  upon  the  stone.  This  brush  is  made  upon  a  short  tube 
fitted  into  the  end  of  the  metal  tube.  To  make  this  brush,  first 
cover  the  plain  part  of  the  small  brass  tube  with  thick  shellac 
dissolved  in  absolute  alcohol.  Place  a  layer  of  the  bristles  around 
it  and  wrap  them  tightly  with  a  fine,  strong  thread.  Then  place 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          411 

more  shellac  over  this  and  another  layer  of  bristles.  Continue  this 
until  the  brush  is  large  enough.  Then  wrap  thoroughly  with  a 
cord  in  shellac,  let  it  dry,  and  then  trim  it  up.  Two  of  these  have 
served  for  four  years  of  fairly  hard  usage. 

Waste  Water. — A  spatter  guard  is  made  by  bending  a  f-inch 
round  brass  tube  into  a  circle,  the  inner  diameter  of  which  is  the 
size  of  the  stones  used,  and  brazing  the  ends  together  solidly. 
Then  this  is  fixed  in  the  lathe  and  one-fourth  of  its  inner  circular 
diameter  is  turned  away.  The  grinding  stones  will  then  go  inside 
this.  Then  this  piece  is  provided  with  a  foot  and  hollow  post  and 
fitted  to  the  lathe  bed  with  a  washer  and  nut,  the  same  as  other 
pieces  are  attached.  This  catches  all  waste  water  and  through  a 
rubber  tube  attached  to  the  end  of  its  hollow  post  under  the  lathe 
bed  delivers  it  into  a  receptacle  so  placed  by  the  table  as  to  receive 
it.  This  prevents  all  of  the  spattering  of  water  which  would  be 
thrown  from  a  rapidly  revolving  wheel  without  it.  If  it  should 
be  inclined  to  run  over  when  a  very  full  stream  is  wanted,  a  piece 
of  rubber  dam  may  be  stretched  over  the  foot  and  pulled  to  its 
upper  end.  This  may  be  caught  under  the  guard  in  fastening  it 
to  the  lathe  bed,  and  will  deliver  any  overflow  into  a  receptacle 
placed  to  receive  it.  In  this  way  nothing  is  wet  or  spattered  with 
water. 

Preparation  of  Material. — In  the  preparation  of  material,  such 
as  teeth,  bone,  etc.,  in  histological  work  of  ordinary  delicacy,  the 
specimen  is  first  ground  flat  on  one  side  by  hand  on  a  rough  stone 
4  inches  in  diameter,  on  the  motor,  and  finished  perfectly  flat  on 
one  of  the  finer  stones  on  the  lathe  head.  The  piece  is  then  washed 
clean  and  placed  in  absolute  alcohol  for  a  sufficient  time  to  remove 
all  traces  of  water,  or,  when  cracking  or  injury  from  shrinkage  is 
not  feared,  it  may  be  dried  in  the  warming  box.  Then  when  dried 
and  warmed  to  about  120°  F.,  it  is  ready  to  mount  with  balsam  on 
the  grinding  disk  for  grinding. 

Management  of  Balsam. — I  suppose  the  management  of  balsam 
will  always  be  a  difficult  problem  with  many  persons.  Many, 
however,  learn  it  quickly.  One  may  take  the  dry  balsam  and 
dissolve  it  in  xylol,  and  filter  it  at  a  high  temperature,  say  110° 
or  120°  F.  Or  one  may  use  the  prepared  balsam  for  microscopic 
mountings.  In  either  case  it  must  be  evaporated  until  stiff  enough 
so  that  it  will  move  rather  sluggishly  at  110°  F.,  but  will  be  fluid 
at  120°  or  130°  F. 


412 


APPENDIX  CHAPTER  I 


Spiders  and  Dogs. — For  using  this  another  bit  of  apparatus  is 
necessary.  A  circular  piece  of  steel  made  flat  on  the  upper  surface 
is  mounted  on  three  legs  1|  to  2  inches  high.  The  steel  disk  should 
have  two  rows  of  holes  around  its  periphery,  the  one  row  f  inch 
inside  the  other.  A  hard-rolled  tool-steel  wire,  or  rod  -^  inch  in 


FIG.  340.— The  spider  with  a  grinding  disk  upon  it  and  a  specimen  laid  on  and 
secured  by  bent  rods  called  dogs.  When  these  dogs  are  placed  and  pressed  down 
through  the  holes  in  the  disk  of  the  spider,  they  hold  fast.  With  a  little  pressure 
of  the  finger  outward  on  the  end  of  the  rod  below  the  disk  of  the  spider,  the  dog 
slips  up  and  is  loose.  The  disk  of  the  spider  is  three  inches  in  diameter. 

diameter,  should  exactly  fit  these  holes.  These  rods  should  now 
be  bent  at  right  angles  with  a  short  nib  on  the  end,  bent  again  at 
right  angles,  so  that  it  will  point  downward  when  the  free  end  of 
the  rod  is  set  into  one  of  the  holes.  The  length  between  these  two 
angles  should  vary  from  f  to  1|  inches  in  three  dozen  or  more 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          413 

pieces  which  should  be  prepared.  The  end  which  goes  in  the 
holes  should  be  cut  so  that  it  will  not  quite  reach  the  surface  of 
the  table  when  dropped  into  the  holes  with  the  end  of  the  nib  on 
the  surface  of  the  circular  plate.  These  rods  are  called  "dogs" 
(Fig.  340). 

With  this  arrangement  a  warming  box  arranged  with  a  thermo- 
stat to  maintain  an  even  temperature,  sufficiently  high  to  soften 
the  stiff  balsam,  is  used.  The  specimen,  the  balsam,  the  grinding 
disk,  and  the  "spider"  are  placed  inside,  and  allowed  to  rest  until 
they  have  reached  the  temperature  desired.  Then  working  quickly, 
a  sufficient  amount  of  balsam  is  placed  on  the  grinding  disk,  and 
the  specimen  laid  on  it.  This  should  be  pressed  down  until  it  is 
seen  that  all  space  under  it  is  filled  with  balsam,  but  no  considerable 
excess  should  be  used.  It  is  well  if  this  rest  so  in  the  warming 
box  for  fifteen  minutes  for  the  balsam  to  soak  well  into  the  speci- 
men. Then  the  grinding  disk,  with  the  specimens,  should  be  laid 
on  the  spider  and  one  of  the  dogs  dropped  into  one  of  the  holes 
in  the  steel  plate,  that  will  bring  its  nib  on  to  a  part  of  the  speci- 
men chosen.  Then  another,  and  still  another,  should  be  placed, 
each  with  its  nib  on  a  different  part  of  the  specimen,  so  that  every 
part  of  it  may  be  pressed  flat  on  the  disk.  More  dogs  should  be 
added  if  necessary.  Now  each  in  turn  is  pressed  down  a  little, 
one  after  another,  until  all  are  exerting  about  all  the  force  the 
spring  of  the  rods  will  exert  without  permanently  bending  them. 
In  this  condition  the  whole  thing  is  again  enclosed  in  the  warming 
box. 

At  this  time  any  number  of  specimens  of  teeth  or  bits  of  teeth, 
bone,  etc.,  that  the  face  of  the  disk  will  hold  may  be  placed  on  the 
disk,  and  all  may  be  ground  together.  Four  to  six  lengthwise 
sections  of  incisor  or  cuspid  teeth  may  be  placed  at  once,  or  eight 
to  twelve  cross-sections.  It  seems  to  be  best  practice,  however, 
not  to  load  the  disk  too  heavily.  Four  lengthwise  sections  will 
grind  better  than  six,  as  a  rule. 

Now,  after  the  loaded  disk  had  remained  in  the  warming  box 
until  all  balsam  that  will  come  has  been  squeezed  out  from  under 
the  specimens,  all  excess  of  balsam  should  be  very  carefully  removed, 
or  wiped  away,  close  up  against  the  specimens.  Nothing  clogs  a 
stone  and  stops  its  cutting  more  effectually  than  balsam  smeared 
over  it,  and  every  excess  that  may  come  against  the  stone  should 
be  got  out  of  the  way. 

When  this  is  done  the  whole  thing  should  be  returned  to  the 


414  APPENDIX  CHAPTER  I 

warming  box  for  from  one  to  four  hours,  so  that  it  may  dry  some 
about  the  margins  at  least.  Then  it  may  be  removed  from  the 
warming  box  and  allowed  to  cool,  and  await  convenience  in  grind- 
ing. It  should,  however,  remain  secured  on  the  spider  by  the 
dogs  if  it  is  to  wait  more  than  a  few  hours,  for  the  disposition 
of  dentin  to  warp  in  drying  may  pull  some  part  of  the  specimen 
from  the  disk.  Under  these  conditions,  two  or  three  days,  or  a 
week,  will  do  no  harm. 

When  the  grinding  is  completed,  the  disk  is  removed  from  the 
machine  and  the  specimens  flushed  with  clean  water,  and  dried  by 
the  pressure  of  a  soft  napkin  folded  to  several  thicknesses,  or  clean 
pieces  of  waste  cotton  fabric  may  be  used.  Then  the  disk  with 
its  specimens  should  be  laid  in  a  dish  and  sufficient  xylol  added 
to  cover  it,  and  allowed  to  rest  until  the  balsam  has  been  dissolved 
and  the  specimens  released.  This  will  usually  require  from  twenty 
to  thirty  minutes,  or  sometimes  as  much  as  an  hour.  When  the 
specimens  are  very  thin  they  loosen  much  quicker  than  when 
thick.  Any  material  not  penetrated  by  xylol,  as  silicified  petri- 
factions and  stones,  require  much  more  time. 

When  the  specimens  have  loosened,  they  are  ready  for  permanent 
mounting  for  microscopic  study. 

Rapidity  of  Grinding. — In  order  to  make  rapid  progress  in  grinding 
specimens,  one  should  have  six  to  ten  grinding  disks,  nearly  as 
many  spiders,  and  a  large  supply  of  dogs.  The  machine  is  so 
nearly  automatic  in  its  action  that  it  needs  but  little  watching, 
so  that  the  preparation  may  be  going  on  while  the  grinding  is  in 
progress.  One  of  the  principal  points  that  needs  attention  is  the 
flow  of  water.  But  if  the  water  and  ice  placed  in  the  receptacle 
are  clean  and  free  from  dirt  or  trash  that  may  stop  the  flow  of 
water,  the  only  care  is  that  the  quantity  of  water  is  kept  up.  The 
vessel  should  be  large  enough  to  hold  a  supply  for  several  hours. 
If  the  stone  should  run  dry,  the  specimen  would  be  destroyed  in 
a  few  seconds. 

Setting  the  Measurement  of  Grinding  Disks. — When  beginning 
any  considerable  series  of  grindings,  the  first  thing  of  importance 
is  to  try  out  and  obtain  a  record  of  the  measurements  of  each 
grinding  disk  for  the  particular  stone  that  may  be  selected  for 
finishing.  I  find  that  most  persons,  after  some  practice,  prefer 
to  use  a  fine  stone  for  the  entire  grind.  In  grinding  teeth,  after 
roughing  down  the  surface  that  is  to  form  the  specimen,  the  back 
is  also  ground  away  to  a  flat  surface  that  will  better  accommodate 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          415 

the  placing  of  dogs  in  mounting  on  the  grinding  disks.  These  may 
be  made  quite  thin  and  reduce  the  grinding  with  the  fine  stone. 
Then  the  stone  selected  is  placed  in  the  lathe  head,  seeing  to  it 
carefully  that  the  face  of  the  stone  is  clean.  Then  the  grinding 
machine  is  brought  up  in  contact  with  the  set-screw  of  the  point- 
finder.  The  tail-piece  is  placed  in  position  and  pushed  up  so  as  to 
make  some  pressure  on  the  shaft.  Then,  with  the  large  nut  the 
shaft  is  so  adjusted  that  the  grinding  disk  being  tried  comes  close 
to  the  stone  but  does  not  touch  it.  Now  start  the  machine  and 
note  the  running  carefully,  and  while  doing  so  catch  the  adjusting 
nut  of  the  micrometer  and  move  it  one-thousandth  at  a  time,  and 
listen  for  the  first  touch  of  the  disk  to  the  stone.  The  moment 
this  is  heard,  quickly  reverse  the  movement  of  the  adjusting  nut 
and  separate  the  disk  from  the  stone.  Try  this  again  and  again, 
until  you  feel  very  certain  of  having  detected  the  first  touch  of 
the  stone  on  the  disk  by  moving  the  adjusting  nut  half  or  a  quarter 
of  ToVo"  inch.  At  last,  while  it  is  touching,  stop  the  machine  in  a 
position  to  see  the  finger  on  the  adjusting  nut,  and  read  the  measure- 
ment and  enter  it  on  your  record  for  that  disk.  In  setting  for  a 
grind  with  this  disk,  turn  the  adjusting  nut  so  as  to  draw  the 
grinding  disk  back  from  the  stone  T"oVo  inch.  When  the  specimens 
to  be  ground  are  mounted  on  this  disk,  place  it  back  on  the  machine, 
start  it,  seeing  that  the  iced  water  is  running  first,  and  let  it  run 
until  it  ceases  to  cut,  which  it  will  do  when  the  forward  movement 
of  the  shaft  is  stopped  by  the  contact  of  the  adjusting  nut  of  the 
micrometer  with  the  rear  bearing  of  the  shaft. 

Then  remove  the  disk  and  examine  the  specimens  carefully.  If 
the  placement  has  been  accurate,  the  specimens  will  be  too  thick. 
Replace  the  disk  carefully  and  turn  the  nut  forward  so  as  to  grind 
one-thousandth  of  an  inch  thinner,  or  one  may  do  only  a  half  of 
one-thousandth  at  a  time.  Repeat  this  until  the  section  seems  to 
be  thin  enough.  Then  remove  and  mount  the  sections  and  judge 
them  with  the  microscope.  By  this  time  one  will  have  arrived  at 
an  accurate  measurement  of  this  disk,  and  the  record  will  be  trust- 
worthy for  other  grinds,  and  will  not  have  to  be  repeated  until 
the  wearing  of  the  stone  begins  to  leave  the  specimens  a  bit  thick. 
Then  a  half-thousandth  of  an  inch  will  bring  it  right.  And  so  on, 
and  on.  Each  disk  will  be  treated  in  the  same  way  for  each  stone 
used,  and  if  one  is  doing  much  grinding  all  will  be  running  on  their 
records,  and  all  go  smoothly.  Recently  a  man  who  was  grinding 
sections  of  teeth  for  me  made  all  of  the  preparations,  preparatory 


416  APPENDIX  CHAPTER  I 

grindings,  and  disk  mounts,  ground  and  removed  from  the  disks 
ready  for  mounting  forty  full-length  sections  of  central  incisors 
in  six  hours,  and  had  his  lunch  during  the  time.  Every  section  was 
complete,  was  even  in  thickness  in  every  part,  and  all  practically 
the  same  thickness — a  thickness  chosen  for  the  special  studies  in 
hand. 

Grinding  Frail  Material. — While  the  machine  facilitates  the 
production  of  the  more  ordinary  sections  to  such  a  degree  as  to 
be  indispensable  to  one  having  many  grindings  to  do,  it  is  in 
the  production  of  sections  of  very  frail  material  that  the  grinding 
machine  stands  out  as  vastly  superior  to  other  methods  of  grinding. 
In  the  study  of  caries  of  enamel  in  which  disintegration  has  ren- 
dered the  remaining  tissue  very  frail  and  likely  to  fall  to  pieces 
before  it  is  sufficiently  thin,  we  may  obtain  the  required  thinness 
and  yet  retain  all  of  the  tissue.  I  have  also  produced  exceedingly 
fine  sections  of  salivary  calculus,  and  equally  good  sections  from 
small  crumbs  of  serumal  calculus.  The  production  of  these  is 
slow,  but  fairly  certain  of  good  results. 

Also  in  grinding  sections  of  fossil  teeth,  fossil  woods,  and  the  like, 
in  which  very  fine  sections  are  too  brittle  to  be  handled  in  any 
way  except  as  stuck  to  glass,  the  machine  gives  excellent  results. 
In  geological  work  it  practically  removes  the  difficulties.  Good 
sections  of  the  very  brittle  stones  can  be  made  with  fair  safety  by 
grinding  on  the  cover-glass. 

Plans  for  Grinding  Frail  Material. — Much  very  desirable  material 
for  microscopic  investigation  will  be  found  that  is  so  frail,  or  at 
leact  so  brittle,  when  reduced  to  sections  thin  enough  for  microscopic 
investigation,  that  it  will  crumble  to  pieces,  either  in  the  grinding 
or  in  the  mounting,  by  the  ordinary  processes.  For  grinding  and 
mounting  such  material  the  following  processes  have  been  slowly 
evolved.  These  may  be  divided  into  the  balsam  process  and  the 
shellac  process.  Such  material  that,  when  made  fast  to  a  cover- 
glass  and  ground  in  hard  balsam,  is  not  liable  to  go  to  pieces  when 
this  hard  balsam  is  softened  by  sticking  the  specimen  and  glass 
cover  to  a  glass  slide  may  be  ground  in  hard  balsam.  If,  however, 
the  different  parts  are  liable  to  separate  and  change  position  when 
the  balsam  softens,  shellac  should  be  used  for  the  grinding.  I  have 
had  some  very  sorrowful  failures  in  grinding  rare  specimens  of 
enamel  that  had  no  cementing  substance  betwreen  the  enamel  rods 
in  hardened  balsam.  For  when  the  softer  balsam  was  added  to 
mount  the  specimen  on  the  glass  slide,  the  hard  balsam  was  softened 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS  417 

and  the  enamel  rods  floated  out  of  position.  All  such  material  as 
will  not  hold  together  strongly  enough  to  prevent  this  should  be 
ground  in  shellac. 

To  grind  in  hard  balsam,  the  one  side  of  the  specimen  may  be 
ground  flat  on  the  rough  stone  and  then  dried  out  in  absolute 
alcohol.  Then  the  ground  side  should  be  saturated  to  sufficient 
depth  with  soft  balsam,  and  laid  aside  until  the  balsam  has  become 
hard  enough  to  grind  smoothly.  Then  the  grinding  and  polishing 
of  this  first  side  should  be  completed  by  grinding  away  all  balsam 
from  the  immediate  surface,  and  sufficiently  into  the  substance 
of  the  specimen  to  produce  a  clean,  smooth  surface  of  the  material. 
When  this  has  been  done,  and  the  surface  dried,  it  should  be 
mounted  on  an  ordinary  cover-glass,  the  thickness  of  which  should 
have  been  measured  and  recorded.  In  this  mounting  the  cover- 
glass  should  be  laid  on  a  spider  and  weight  enough  placed  upon  it 
to  insure  a  perfect  fit  of  the  surface  of  the  glass.  This  should  be 
subjected  to  about  120°  F.  heat  for  from  one  to  five  or  six  hours, 
for  the  purpose  of  expressing  the  last  bit  of  balsam  possible  from 
between  the  specimen  and  the  cover-glass.  Then  it  may  rest, 
awaiting  the  convenience  of  the  operator,  for  several  days,  but  the 
balsam  must  not  be  allowed  to  become  "brittle  hard,"  because  in 
that  case  it  loses  toughness.  All  excess  of  balsam  about  the  margins 
of  the  specimen  should  be  carefully  removed  to  facilitate  the 
hardening  of  that  which  remains,  and  especially  so  that  it  may  not 
come  in  contact  with  the  grinding  stone,  stick  to  its  surface,  and 
interfere  with  the  cutting. 

Good  judgment  must  be  acquired  by  practice  as  to  the  hardening 
of  balsam  and  shellac  in  these  grinding  processes.  The  best  idea 
of  it  that  can  be  given  in  words  is  this.  The  balsam  or  the  shellac 
must  have  become  firm  enough  so  that  it  will  not  drag  or  allow  the 
specimen  to  move  while  grinding  in  iced  water.  Neither  must  it 
become  hard  enough  to  become  brittle,  /or  then  it  becomes  liable  to 
break. 

When  ready,  the  specimen  is  mounted  on  the  grinding  disk. 
This  is  done  by  first  cleansing  the  disk,  finishing  with  xylol,  and 
then  sealing  the  cover-glass  to  this  with  soft  balsam.  This  should 
be  placed  on  the  spider  and  well  weighted  down  with  dogs.  All 
excess  of  balsam  should  be  carefully  wriped  away  from  the  margins 
of  the  cover-glass.  This  may  be  quickly  dried  at  120°  F.,  or  more 
slowly  at  room  temperature.  It  should,  however,  be  warmed  for 
a  half-hour  or  more,  for  the  purpose  of  expressing  as  much  balsam 
27 


418  APPENDIX  CHAPTER  I 

as  possible.  This  cover-glass  will  be  well  held  for  grinding  in 
iced  water  with  only  a  little  drying  about  the  margins,  if  all  excess 
of  balsam  is  cleaned  away  closely.  The  balsam  should  not  become 
very  hard. 

If  the  specimen  is  of  considerable  bulk  and  of  a  quality  of  material 
that  can  be  cut  with  a  steel  saw,  the  disk  may  be  caught  in  a  vise 
"with  leather-cushioned  jaws  to  avoid  bruising,"  and  the  bulk 
of  the  material  removed  with  a  jeweler's  saw,  leaving  only  a 
moderately  thin  section  for  grinding.  Or  if  the  material  is  very 
hard,  as  stones,  silicified  fossils,  etc.,  the  disks  may  be  mounted 
upon  the  slide  rest  and  cut  with  the  slicing  disks,  to  be  described 
later. 

The  specimen  is  now  ready  for  the  final  grinding.  The  record 
for  measurement  with  the  particular  stone  to  be  used  in  finishing 
has  been  made,  tried  out  on  unimportant  material,  and  the  cover- 
glass  has  been  measured  and  its  record  made.  With  this  data, 
the  disk  is  screwed  to  its  place,  the  micrometer  turned  to  the 
proper  measurement  for  the  finish,  the  iced  water  arranged,  the 
machine  set  in  motion,  and  it  will  do  the  rest.  When  coarser 
stones  are  used  for  cutting  away  considerable  material,  I  find  those 
with  just  a  little  experience  prefer  to  gauge  the  amount  of  the 
cutting  by  the  eye  for  the  coarse  stone. 

Removal  of  the  Cover-glass  from  the  Disk. — I  remove  the  cover- 
glass  with  the  specimen  from  the  grinding  disk  in  two  different 
ways,  as  seems  at  the  time  best. 

First,  the  grinding  disk  is  placed  on  a  heated  piece  of  metal 
that  will  warm  the  grinding  disk  quickly.  Have  a  stick  of  rather 
soft  wood  ready,  the  end  of  which  is  cut  to  a  rather  sharp  angle 
and  thinned  down  almost  in  the  form  of  a  blade.  When  the  grind- 
ing disk  begins  to  warm,  catch  the  margin  of  the  cover-glass  with 
the  end  of  the  stick  and  begin  to  make  steady  pressure.  As  the 
disk  warms,  so  as  to  soften  the  balsam,  the  cover-glass  will  begin 
to  move  under  the  steady  pressure,  slowly  at  first,  but  more  rapidly 
later,  and  will  slide  off  the  grinding  disk  before  the  specimen  is 
loosened.  For  this  plan  the  cover-glass  should  be  pretty  strong, 
one  and  one-half  to  two  thousandths  of  an  inch  thick.  Otherwise 
there  will  be  great  danger  of  breaking  it.  It  is  well  in  some  cases 
to  run  just  a  little  xylol  around  the  margins  of  the  cover-glass 
and  partially  dissolve  the  balsam  that  has  become  driest  before 
the  heating.  Great  care  must  be  taken  not  to  allow  the  xylol  to 
spread  on  to  the  specimen,  for  it  would  loosen  it  very  quickly. 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          419 

The  specimen  is  then  turned  downward  and  placed  on  a  tiny 
drop  of  balsam  on  a  glass  slide,  and  quickly  pressed  down  close 
and  level.  As  the  new  balsam  will  soften  the  old,  it  should  not  be 
moved  further  than  to  quickly  apply  a  light  spring  clip  to  hold  it 
steady.  The  parts  of  the  specimen  are  less  likely  to  move  if  this 
is  laid  on  ice  for  an  hour  or  more. 

The  Use  of  Shellac. — In  the  second  plan  shellac  is  used  instead 
of  balsam  for  hardening  the  specimen  and  holding  its  parts  together 
in  the  first  grinding.  This  part  of  the  work  is  otherwise  done  in 
the  same  way.  The  drying  of  the  shellac  requires  more  time  usually 
than  the  balsam. 

The  attachment  of  the  cover-glass  to  the  grinding  disk  is  done 
in  the  same  way  as  when  balsam  is  used  to  hold  the  specimen  on 
the  cover-glass — that  is,  with  balsam.  The  grinding  proceeds 
similarly  in  every  respect. 

In  the  removal  of  the  cover-glass  from  the  grinding  disk,  and 
mounting  the  specimen,  comes  the  important  differences  in  the 
two  processes.  Xylol  dissolves  balsam  very  quickly.  But  xylol 
does  not  dissolve  shellac  at  all.  Therefore,  instead  of  pushing  the 
cover-glass  of  the  grinding  disk,  the  disk  is  laid  in  xylol  and  the 
balsam  dissolved  out.  In  this  there  is  no  danger  of  detaching  or 
moving  the  specimen  if  the  handling  is  careful.  When  cleaned,  it 
is  inverted  upon  a  glass  slide  on  a  drop  of  balsam  without  fear  of 
movement  of  parts  of  the  specimen,  no  matter  how  frail. 

The  Preparation  of  Shellac. — To  keep  shellac  in  condition  for 
this  work  has  some  difficulties.  The  dry  scales  should  be  dis- 
solved in  absolute  alcohol  so  as  to  make  a  moderately  thick  varnish. 
It  should  then  be  filtered  at  a  temperature  of  110°  to  120°  F.,  or 
be  made  thinner  and  filtered  at  room  temperature.  Great  care 
should  be  exercised  to  keep  the  filtrate  from  exposure  to  a  damp 
atmosphere,  for  it  absorbs  water  readily  and  then  will  throw  down 
fine  crystals,  which  destroy  its  value  for  microscopic  purposes. 

After  being  filtered  it  should  be  evaporated  in  a  close  warming 
box  in  about  110°  to  120°  F.,  to  the  consistence  of  syrup.  In  doing 
this  it  is  well  to  divide  the  supply  into  two  or  three  grades — a 
thinner,  medium,  and  a  thicker  solution.  The  thinner  solution 
will  be  used  for  saturating  frail  specimens  before  any  cutting  is 
done.  The  thicker  solutions  for  attaching  specimens  to  the  cover- 
glass  for  grinding.  The  medium  solution  for  either  purpose,  as  the 
material  may  seem  to  require. 


420  APPENDIX   CHAPTER  I 

The  Grinding  from  Crumbled  Material. — There  is  often  important 
material  for  investigation  that  can  be  had  only  in  very  small  crumbs, 
or  broken  pieces,  such  as  serumal  calculus,  sands,  crumbled  bits 
of  strange  stones,  or  mixtures  of  such  material  as  is  found  in  some 
of  the  coarser  sands.  These,  on  microscopic  investigation,  may 
tell  important  stories  as  to  their  origin  and  throw  important  light 
upon  geological  questions.  In  addition  to  the  ordinary  microscopic 
observation,  the  polariscope  may  be  turned  on  these,  and  reveal 
important  facts  as  to  their  origin  and  structure.  Also  many  things 
will  be  found  in  botanical  work,  such  as  obtaining  sections  of  small 
seeds,  and  the  like,  which  will  give  important  information. 

Having  done  a  few  of  these  grindings,  especially  of  the  very 
frail  dental  material,  such  as  serumal  calculus,  extremely  frail 
fossil  teeth,  etc.,  plans  of  work  more  or  less  well  adapted  have  been 
developed. 

For  instance,  I  have  obtained  excellent  sections  of  serumal 
calculus,  which  can  be  had  only  in  small  crumbs  or  flakes,  in  this 
wise:  A  small  collection  of  these  bits  are  first  immersed  for  a 
time  in  absolute  alcohol,  or  until  all  air  has  been  removed  if  they 
are  dry,  or  if  they  are  freshly  gathered,  until  all  water  has  been 
removed.  Then  a  cover-glass  is  prepared  by  covering  its  central 
part  with  the  thicker  solution  of  shellac,  and  these  crumbs  are 
placed  in  this,  in  what  seems  to  be  the  best  position  for  obtaining 
sections.  These  are  allowed  to  soak  full  of  the  shellac,  under  a 
close  cover,  and  then  uncovered  to  dry  up.  Then,  if  some  of  the 
pieces  seem  to  need  it,  more  shellac  is  added  from  time  to  time, 
until  the  embedding  seems  sufficient.  This  may  be  dried  at  room 
temperature,  or  in  the  warming  oven  at  110°  to  120°  F.  Shellac 
should  not  be  subjected  to  much  higher  temperatures  for  a  con- 
siderable time,  because  continued  high  temperature  for  many  days 
together  seems  to  injure  the  strength. 

When  this  is  sufficiently  hard  for  smooth  grinding,  and  before 
it  has  become  too  brittle  (determining  this  point  requires  some 
experience),  the  preparation  is  cemented  to  the  grinding  disk  with 
balsam  and  ground  to  such  a  point  as  seems  most  favorable  for 
obtaining  sections.  This  point  is  to  be  determined  by  frequent 
removal  of  the  disk  from  the  machine  and  examination  of  the 
exposed  surfaces  of  the  several  pieces. 

When  this  part  is  done,  the  cover-glass  is  dissolved  off  of  the 
grinding  disk  by  xylol.  Then  another  cover-glass  is  attached  to 
the  surface  with  the  least  possible  amount  of  shellac.  This  in  turn  is 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS          421 

dried  to  the  right  consistence.  Then  the  last  cover-glass  placed — 
that  is,  the  one  on  the  side  that  has  been  ground — is  secured  to  the 
grinding  disk  with  balsam.  When  this  has  set  it  is  placed  on  the 
machine  and  the  first  cover-glass  is  ground  away  and  the  section 
ground  to  the  required  thinness.  They  are  again  dissolved  off  of 
the  grinding  disk,  and  may  be  at  once  mounted  in  balsam  on  the 
microscopic  slide. 

Difficulties  in  Grinding. — In  the  grinding  of  material  enveloped 
in  shellac,  or  in  balsam,  either  of  these  materials  are  apt  to  gum  up 
the  stone  and  stop  the  cutting,  or  render  the  grinding  very  slow. 
When  this  is  from  balsam,  it  may  be  quickly  removed  after  drying 
the  stone  by  washing  with  xylol  on  a  brush,  or  a  bit  of  cloth,  while 
the  stone  is  slowly  revolved. 

When  clogged  with  shellac,  the  washing  is  done  with  absolute 
alcohol.  This  requires  much  more  time,  and  some  advantage 
may  be  obtained  by  using  pumice  stone  with  the  cloth  or  with 
cork.  After  rubbing  with  pumice  stone,  a  very  thorough  washing 
with  alcohol  should  be  made  to  remove  the  last  particles  of  pumice, 
before  rebeginning  the  grinding.  Even  with  this,  the  ground 
surface  is  apt  to  be  rough  or  scratched  for  a  time  by  particles  of 
the  pumice  lodged  on  the  stone.  These  will  soon  disappear,  how- 
ever. Yet  the  pumice  should  not  be  used  in  the  last  portion  of  the 
grinding. 

With  much  grinding  of  hard  substances,  the  surfaces  of  the 
stones  become  worn  so  smooth  that  they  do  not  cut  wTell.  Then 
the  picking  tool  should  be  run  over  the  surface  until  it  is  perceptibly 
roughened.  This  will  cause  the  stone  to  cut.  briskly  for  a  con- 
siderable time,  and  at  first — following  such  sharpening — the 
ground  surface  of  the  specimen  is  likely  to  be  full  of  scratches. 
In  that  case  a  smooth  stone  should  be  used  for  the  finishing. 

Much  care  should  be  taken  in  keeping  the  stones  in  good  condi- 
tion. Except  in  the  ways  mentioned,  no  dirt  or  grit  should  be 
allowed  to  come  in  contact  with  their  surfaces.  A  single  particle 
of  grit  lodged  in  the  surface  of  the  stone  will  fill  the  whole  surface 
of  the  ground  section  with  scratches.  Although  I  shut  up  my 
stones  in  a  close-fitting  drawer,  I  find  it  necessary  to  cover  each 
with  a  close-fitting  cloth  that  is  so  closely  woven  as  to  exclude 
all  dust. 

In  taking  care  of  the  machine  itself,  one  cannot  be  too  careful. 
All  of  the  bearings  of  the  lathe  head  and  of  the  grinding  machine 
should  be  swaddled  with  candle  wick  saturated  with  oil  to  prevent 


422  APPENDIX  CHAPTER  I 

the  ingress  of  gritty  particles.  This  is  especially  needful  when 
using  the  aluminum  saws  and  feeding  them  with  carborundum 
powder.  Then  every  bearing  about  the  whole  machine  should  be 
especially  protected  to  prevent  the  possibility  of  getting  grit  in 
the  bearings.  Carelessness  in  such  a  matter  will  quickly  ruin  a 
fine  bit  of  mechanism.  But  with  this  care,  such  a  machine  should 
continue  to  do  its  work  well  for  a  lifetime  (Figs.  341  and  342). 

FIG.  341 


FIGS.  341  and  342. — Arrangement  for  slicing  very  hard  material.  Fig.  341  is  the  more 
ordinary  view  of  the  machine  with  the  slide  rest  and  object  holder  in  position.  In 
Fig.  342  the  lathe  is  turned  about  to  give  a  better  view  of  the  slide  rest,  object  holder, 
spatter  guard,  and  aluminum  disk.  In  these  illustrations  the  slotted  tube  is  used 
(see  text)  to  hold  the  object  being  cut.  Notice  that  the  disk  used  for  cutting  is  sur- 
rounded by  a  spatter  guard  which  is  open  for  a  space  at  one  side  so  that  the  periphery 
of  the  disk  may  be  used  in  cutting.  This  guard  gathers  all  water  and  grit  used  in 
cutting,  and  delivers  it  into  the  pan  below  through  its  hollow  post.  When  doing 
this  kind  of  work  all  of  the  bearings  of  the  machine  should  be  carefully  wrapped 
(swaddled)  to  keep  them  safe  from  intrusion  of  grit. 

The  Slicing  Mechanism. — This  is  an  arrangement  for  slicing 
very  hard  substances  which  cannot  be  cut  with  the  ordinary  steel 
saw — such  as  the  enamel  of  teeth,  silicified  fossils,  rocks,  etc.  This 
consists  of  an  aluminum  disk  fitted  to  the  lathe  head,  and  sur- 
rounded by  a  special  form  of  spatter  guard  that  admits  of  the 
use  of  the  periphery  for  cutting,  and  an  object  holder  fixed  upon 
the  slide  rest  of  the  lathe.  The  object  holder  consists  of  a  clamp 
that  grasps  a  brass  tube  slotted  at  the  free  end  in  which  teeth,  or 
other  objects  may  be  made  fast  with  plaster  of  Paris  or  sealing  wax 
for  slicing.  Or  in  place  of  this  a  brass  mandril,  upon  the  end  of 


THE  GRINDING  OF  MICROSCOPIC  SPECIMENS 


423 


which  there  is  a  threaded  nipple  by  which  any  of  the  grinding 
disks  may  be  attached.  These  are  fixed  in  the  position  of  the 
ordinary  tool  post,  and  maybe  swung  horizontally  to  any  possible 
position  in  relation  to  the  aluminum  disk.  An  object  can  there- 
fore be  so  placed  on  the  disk  as  to  be  cut  in  any  direction  desired. 
Usually  these  are  fixed  upon  the  disk  with  sealing  wax.  In  using 
the  aluminum  disk  it  is  fed  with  carborundum  po\vder  suspended 
in  soapy  water  to  give  it  some  stickiness.  This  is  applied  with 
a  brush  by^hand,  and  is  kept  going  so  constantly  as  to  prevent  the 
disk  from  running  dry.  The  ordinary  aluminum  plate,  of  twenty- 

FIG.  342 


four  to  thirty  gauge,  may  be  used  for  making  these.  They  are  first 
cut  in  circles  by  hand,  as  large  as  the  lathe  will  swing  (4  inches), 
and  then  are  cut  down  to  3^  inches  with  a  tool  in  the  slide  rest. 
These  are  quickly  made  when  wanted.  They  wear  out  rapidly, 
and  yet  one  of  them  will  do  much  cutting  of  very  hard  substances, 
and  do  it  accurately  and  delicately.  Rings  may  readily  be  cut 
from  the  ordinary  test-tubes  without  special  danger  of  breaking. 
The  crown  of  a  molar  tooth  may  be  cut  into  many  slices;  fossil 
teeth,  silicified  fossil  woods,  stones,  etc.,  may  readily  be  sliced  as 
thin  as  they  can  be  handled  in  the  after- work  of  preparation. 


APPENDIX  CHAPTER  II. 


THE   THEORY   OF   HISTOLOGICAL   TECHNIQUE. 

THE  first  requirement  of  histological  technique  is  to  obtain  a 
general  view  of  the  theory  of  procedure.  Many  beginners  make 
the  mistake  of  supposing  that  directions  for  histological  technique 
can  be  followed  like  the  receipts  of  a  cook  book,  or  the  directions 
for  an  experiment  in  chemistry.  This  is  very  seldom  the  case,  and 
while  it  is  always  necessary  to  follow  directions  accurately,  it  is 
still  more  necessary  to  follow  them  intelligently.  All  histological 
methods  require  judgment.  For  instance,  the  length  of  time 
required  for  xylol  to  replace  absolute  alcohol  in  a  block  of  tissue 
which  is  to  be  embedded  depends  upon  the  size  of  the  piece,  the 
character  of  the  tissue,  the  temperature,  and  possibly  some  other 
factors.  It  is  therefore  impossible  to  say  exactly  what  time  would 
be  required,  and  the  experimenter  must  use  the  judgment  which 
has  been  acquired  as  the  result  of  experiment.  In  the  same  way  no 
experimenter  can  make  up  a  stain  and  be  sure  that  it  will  work 
exactly  like  the  last  lot  made  by  the  same  formula  until  he  has 
tried  it.  Even  with  the  same  stain  the  length  of  time  required  for 
staining  a  section  depends  upon  the  thickness  of  the  section,  the 
character  of  the  tissue,  and  the  preliminary  technique  it  has  been 
through.  So  that  all  time  directions  must  be  considered  as  approx- 
imate, and  to  be  successful  the  experimenter  must  study,  first,  the 
object  to  be  obtained  by  the  use  of  each  reagent,  and  the  peculiar 
action  of  the  reagent  upon  the  tissue. 

For  observation  with  the  compound  microscope  transmitted 
light  is  ordinarily  used.  The  object  must  therefore  be  thin  and 
transparent  enough  to  allow  the  light  to  pass  through  it.  The 
higher  the  magnification  the  smaller  the  field,  that  is,  the  smaller 
the  portion  of  the  tissue  that  can  be  seen  at  one  time,  and  the 
less  depth  of  focus,  and  consequently  the  thinner  the  sections 
must  be.  A  section  that  would  be  excellent  for  study  with  the  f 
objective  may  be  almost  valueless  under  a  TV,  and  sections  that 

(424) 


THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE  425 

are  splendid  under  the  T\-  might  be  of  little  value  under  the  f . 
In  other  words,  the  thickness  of  the  section  should  be  related  to 
the  magnification  with  which  it  is  to  be  studied,  and  to  the  size 
of  the  structural  elements  which  make  up  the  tissue.  For  the 
study  of  the  organs  and  tissues  of  multicellular  organisms  there 
are  three  general  methods — (1)  teasing,  (2)  maceration,  and  (3) 
sectioning. 

Teasing. — In  this  method  a  small  portion  of  the  living  tissue 
is  torn  apart  with  two  needles  in  a  drop  of  normal  salt  solution  or 
some  indifferent  medium  which  will  not  affect  the  tissue.  In  this 
way  it  is  spread  into  a  thin  film  and  squeezed  a. little  between 
a  slide  and  cover-glass  so  as  to  separate  the  structural  elements 
when  they  may  be  directly  observed.  Of  course,  in  studying  such 
a  preparation  it  must  be  remembered  that  the  tissue  has  been 
forcibly  torn  apart  and  effects  of  violence  must  be  looked  for. 
These  often  bring  out  facts  of  structure  which  would  not  other- 
wise be  as  easily  seen.  After  teasing  the  living  tissue,  staining 
agents  may  be  used  to  facilitate  the  study  of  structure.  The  fresh 
tissues  are  often  so  transparent  and  made  up  of  substances  of  so 
near  the  same  refracting  index  that  very  little  structure  can  be 
made  out  without  the  use  of  staining  agents.  It  must  be  borne 
in  mind  that  staining  agents  are  of  two  classes,  diffuse  and  selective. 
A  diffuse  stain  gives  an  even  color  to  all  of  the  tissue  and  facilitates 
the  study  chiefly  by  rendering  it  less  transparent.  A  selective 
stain  combines  more  readily  with  one  portion  of  the  tissue  than 
another,  rendering  it  more  conspicuous.  Selective  stains  therefore 
must  be  thought  of  as  chemical  agents  which  combine  with  parts 
of  the  cell  or  tissue  and  demonstrate  chemical  differences  in  the 
structural  elements.  For  instance,  basic  anilines  react  with  the 
chromatin  of  the  nucleus,  producing  a  colored  compound.  The 
stain  may  then  be  washed  out  of  the  section,  leaving  only  the 
nuclei  colored.  Acid  anilines  in  general  are  diffusive  stains  giving 
a  general  color  to  the  cytoplasm.  In  a  similar  way  certain  stains 
will  react  only  or  chiefly  with  intercellular  substances,  rendering 
them  more  conspicuous.  For  staining  freshly  teased  specimens 
methyl  green,  the  formula  for  which  wrill  be  found  under  the  para- 
graph on  stains,  is  an  excellent  agent.  Teased  specimens  are  never 
very  permanent,  though  they  may  be  preserved  for  a  considerable 
length  of  time  by  mounting  in  glycerin  or  glycerin  jelly  and  putting 
a  ring  of  varnish  or  white  lead  around  the  edge  of  the  cover-glass 
so  as  to  prevent  evaporation. 


426  APPENDIX  CHAPTER  II 

Maceration. — When  an  organ  is  composed  of  more  than  one 
tissue  the  structural  elements  may  be  separated  by  selecting  an 
agent  which  will  act  upon  one  and  not  upon  the  others;  for  instance, 
the  muscle  fibers  of  a  voluntary  muscle  may  be  separated  by 
treating  a  piece  of  tissue  with  dilute  alkali,  which  will  soften  and 
dissolve  the  connective  tissue,  allowing  the  muscle  fibers  to  sepa- 
rate. In  a  similar  way  dilute  alcohol  will  soften  the  cementing 
substance  between  the  epithelial  cells.  By  first  treating  a  piece 
of  tissue  with  the  proper  agent  and  then  teasing,  the  form  of  the 
structural  elements  of  the  tissue  can  be  made  out.  By  treating 
a  portion  of  connective  tissue  containing  both  white  and  elastic 
fibers  with  dilute  hydrochloric  or  acetic  acid,  which  dissolves  the 
white  fibers,  elastic  fibers  which  could  otherwise  not  be  seen  may 
be  made  out.  Macerating  and  teasing  methods  are  of  great  assis- 
tance to  the  study  of  tissues  in  sections,  and  it  would  be  often  very 
difficult  to  obtain  true  ideas  of  structure  from  sections  without 
their  assistance. 

Sectioning. — For  the  study  of  the  structural  elements  in  their 
relation  to  each  other  in  the  tissue  sectioning  is  the  one  method. 
As  they  exist  in  the  body,  however,  some  of  the  tissues  are  too  soft 
and  others  too  hard  to  allow  the  cutting  of  a  thin  enough  slice 
without  disturbing  the  relation  of  the  structural  elements.  They 
must  therefore  be  put  through  rather  an  elaborate  process  in  which 
the  object  of  every  step  must  be  understood. 

Dissecting. — First  of  all,  the  material  for  histological  work  must 
be  absolutely  fresh,  that  is,  living.  It  must  be  remembered  that 
living  cytoplasm  is  chemically  different  from  dead  cytoplasm,  and 
as  soon  as  death  occurs  postmortem  changes  begin  which  gradually 
destroy  the  structure.  The  period  from  death  to  the  beginning  of 
histological  methods  of  preparation  should  be  measured  in  minutes, 
not  in  hours.  Tissues  that  have  been  dead  for  a  few  hours  will 
not  react  with  the  staining  agents  so  as  to  produce  the  brilliant 
specimens  that  can  be  obtained  from  fresh  material,  and  often  a 
few  days  will  render  material  entirely  useless  except  for  the  grosser 
anatomical  relations.  The  specimens  to  be  studied  should  be 
dissected  while  the  cells  of  the  tissue  are  still  alive,  and  in  doing 
so  the  greatest  care  should  be  used  not  to  disturb  the  relation  of 
the  tissues. 

Fixing. — Histologically  this  word  means  killing.  After  dissect- 
ing out  the  tissue  to  be  studied,  and  while  the  cells  are  still  alive, 
it  must  be  immersed  in  some  liquid  that  will  kill  the  cells  and  fix 


THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE  427 

their  structure  as  when  alive.  The  pieces  should  be  made  small 
enough  for  the  fixing  agent  to  penetrate  them  rapidly,  and  the  size 
of  the  piece  that  can  be  used  depends  upon  the  density  of  the 
tissue,  its  character,  and  the  nature  of  the  reagent.  Some  fixing 
agents  are  very  much  more  penetrating  than  others.  All  fixing 
agents  coagulate  or  set  the  cytoplasm  and  tend  to  prevent  shrinkage. 
The  success  of  all  the  following  steps  and  the  value  of  the  specimen 
for  the  study  of  detail  of  structure  depend  upon  the  perfection  of 
fixation. 

The  fixing  agents  most  commonly  used  are  bichloride  of  mer- 
cury, potassium  chromate  or  chromic  acid,  osrnic  acid,  alcohol, 
and  formalin.  The  formulas  for  the  same  will  be  found  on  pages 
439  and  441. 

Hardening. — Since  all  the  fixing  agents  coagulate  living  cyto- 
plasm, they  are  also  to  a  greater  or  less  extent  hardening  agents, 
and  after  fixing  tissues  may  be  handled  with  less  danger  of  dis- 
turbing the  relation  of  the  structural  elements.  Some  fixing 
agents,  especially  chromic  fluids,  may  be  continued  in  their  action 
as  hardening  agents  until  the  tissue  has  attained  the  proper  consis- 
tency for  sectioning,  but,  as  a  rule,  it  is  necessary  to  use  other  agents 
for  this  purpose.  In  all  cases  the  fixing  agent  must  be  thoroughly 
washed  out  of  the  tissue  before  the  process  is  continued.  Alcohol 
is  the  universal  hardening  agent,  and  at  the  same  time  it  removes 
the  water  from  the  tissue.  In  carrying  tissues  from  wrater  to  alcohol 
several  grades  must  always  be  used,  and  the  more  delicate  the 
tissue  the  more  gradual  must  be  the  changes.  If  a  piece  of  tissue 
is  taken  from  water  and  placed  in  95  per  cent,  alcohol,  the  diffusing 
currents  will  be  so  strong  as  to  disturb  structure  and  at  the  same 
time  the  hardening  action  is  so  energetic  as  to  produce  shrinkage. 
From  water  a  tissue  should  never  be  placed  in  alcohol  stronger 
than  70  per  cent.,  where  it  should  be  allowed  to  remain  for  twenty- 
four  hours.  From  70  per  cent,  it  may  be  taken  to  95  per  cent, 
for  the  same  length  of  time,  and  from  95  per  cent,  to  absolute, 
which  will  entirely  remove  the  water  and  prepare  the  tissue  for 
embedding.  If  the  tissue  is  very  delicate,  it  should  be  placed  in 
water,  then  in  50  per  cent,  alcohol,  and  carried  through  in  grades 
of  10  per  cent,  to  95  per  cent. 

Embedding. — In  order  to  cut  thin  sections  of  tissue  the  piece 
must  be  surrounded  and  infiltrated  with  some  firm  substance  which 
will  not  only  support  the  entire  piece,  but  will  soak  through  the 
tissue,  filling  all  intercellular  spaces  and  supporting  the  individual 


428  APPENDIX  CHAPTER  II 

structural  elements.  At  the  same  time  the  embedding  material 
is  used  to  fasten  the  tissue  firmly  to  a  block  of  fiber  or  wood  which 
can  be  grasped  in  the  clamp  of  the  sectioning  machine.  Two  kinds 
of  material  are  used  for  this  purpose.  Substances  that  are  fluid 
when  warm,  and  solid  when  cold,  as  paraffin,  or  substances  which 
may  be  dissolved  in  volatile  liquid  and  are  solidified  by  evapora- 
tion, as  celloidin.  In  both  of  these  methods  the  substances,  as  a 
rule,  are  either  oily  or  insoluble  in  water,  and  therefore  the  tissue 
must  be  thoroughly  dehydrated — that  is,  have  all  the  water  removed 
from  it  before  it  is  placed  in  the  embedding  material.  To  accom- 
plish this  there  should  be  at  least  one  change  of  absolute  alcohol. 
From  the  absolute  alcohol  the  tissue  should  be  placed  in  a  fluid 
which  is  a  solvent  for  the  embedding  material,  so  that  it  will  pene- 
trate the  tissue  more  perfectly  and  rapidly.  Heat  is  always  injurious 
to  the  tissue,  and  in  embedding  in  paraffin,  therefore,  the  tissue 
should  be  kept  in  the  melted  paraffin  for  the  shortest  possible  time 
and  paraffin  of  as  low  a  melting-point  as  is  consistent  with  suffi- 
cient hardness  for  cutting  should  be  used.  In  embedding  by 
evaporation  the  evaporation  should  not  be  too  rapid  or  the  shrinkage 
will  be  increased.  Tissues  may  be  kept  blocked  and  ready  to  cut 
for  a  long  time,  but  as  a  general  principle  the  shorter  the  time  the 
more  perfect  will  be  the  specimen. 

Sectioning. — For  sectioning  some  sort  of  machine  is  necessary, 
and  many  kinds  have  been  designed,  the  general  principles  of 
which  are  all  the  same.  They  consist  of  a  clamp  which  holds  the 
knife  and  a  clamp  which  holds  the  specimen,  and  can  be  adjusted 
in  such  a  way  as  to  bring  the  specimen  in  proper  relation  to  the 
knife.  The  position  of  the  specimen  is  advanced  by  a  micrometer 
screw  so  that  sections  of  any  desired  thickness  may  be  sliced.  The 
delicate  part  of  this  machine  is  the  micrometer  screw.  The  essen- 
tial to  the  success  of  its  working  is  the  sharpness  of  the  razor, 
and  for  such  specimens  as  decalcified  bone  the  razor  must  be  heavy 
and  strong,  so  that  the  edge  will  not  spring  in  cutting  the  hard 
tissue. 

Staining. — The  detail  of  staining  process  will  be  described  in 
the  next  chapter,  but  it  must  be  remembered  that  stains,  as  a  rule, 
are  water  solutions  and  the  sections  must  be  carried  through  the 
grades  of  alcohol  to  water  before  they  are  ready  for  the  stain. 
After  staining  they  must  be  carried  back  through  the  grades  of 
alcohol,  so  as  to  remove  the  water  entirely  before  they  can  be 
mounted  in  balsam,  which  is  not  soluble  in  water. 


THE  THEORY  OF  HISTOLOGICAL  TECHNIQUE  429 

Mounting. — Except  in  serial  work,  but  one  specimen  should  be 
placed  on  a  slide,  and  this  should  be  in  the  center,  leaving  room  at 
either  end  for  a  label.  In  serial  work  the  sections  may  be  placed 
at  one  end  of  the  slide,  preferably  the  left  hand,  leaving  room  at 
the  right  for  one  label. 

Labelling. — Nothing  in  histological  technique  is  more  important 
than  labelling,  especially  in  all  research  work.  Through  every 
step  of  the  process  the  specimen  must  be  kept  track  of,  and  a 
mixing  of  labels  may  spoil  months  of  work.  A  laboratory  note-book 
containing  a  record  of  all  material  and  work  should  always  be  on 
the  tables.  I  have  found  a  system  of  date  and  number  convenient. 
For  instance,  on  June  4  a  number  of  specimens  are  dissecte'd  out; 
in  the  note-book  the  record  of  the  source  of  the  tissue  is  made;  the 
first  piece  is  placed  in  a  bottle  of  fixing  fluid  and  the  bottle  labelled 
6-4-1911,  No.  1;  the  second,  6-4-1911,  No.  2,  and  so  on.  In  the 
note-book  the  description  of  each  block  and  the  date  and  the  hour 
when  it  was  placed  in  the  fluid  is  recorded.  In  this  way  the  tissue 
may  be  carried  clear  through  recording  each  step  in  the  process, 
and  when  it  is  sectioned  and  mounted  we  can  follow  its  history  in 
the  note-book.  Every  slide  should  be  labelled  first  with  the  date 
and  the  block  number  so  as  to  follow  its  technique;  second,  the 
name  of  the  tissue,  and  third,  the  kind  of  staining.  This  should 
be  placed  on  the  right-hand  label,  leaving  the  left-hand  label  for 
index  and  file  number  if  the  section  is  preserved. 

Indexing  and  Filing. — Many  beginners  make  the  mistake  of  not 
indexing  and  filing  their  slides.  They  think  because  they  have 
only  a  few,  that  they  can  easily  find  anything  they  want,  and  that 
they  will  wait  until  they  have  a  larger  number  before  they  begin 
a  system,  but  when  a  large  number  have  piled  up  they  can  never 
find  time  to  go  back  and  arrange  them  as  they  should  be.  And 
only  one  who  has  failed  in  this  way  knows  the  annoyance  of  looking 
through  hundreds  of  slides  to  find  one  that  he  knows  he  has  some 
place. 


APPENDIX  CHAPTER  III. 


GENERAL   HISTOLOGICAL  METHODS. 

Fixing.— As  has  been  seen  from  the  preceding  chapter,  fixing  is 
the  first  and  one  of  the  most  important  steps  in  all  histological 
methods.  No  degree  of  care  in  the  latter  steps  can  make  up  for 
any  imperfection  in  it.  As  a  general  statement  all  fixing  agents 
have  advantages  and  disadvantages,  so  that  in  research  work  several 
should  be  tried  and  their  results  compared.  For  class-room  work, 
however,  minute  details  are  not  so  important.  Certain  general 
principles  may  be  stated.  Bichloride  of  mercury  is  especially 
adapted  to  the  fixing  of  epithelium  of  the  mucous  membrane.  It, 
however,  does  not  penetrate  rapidly,  and  small  pieces  must  be 
used.  Crystals  are  liable  to  form  in  the  tissue,  and  special  precau- 
tions must  be  taken  for  their  removal.  Flemming's  and  Zenker's 
fluids  and  the  fluids  containing  osmic  acid  are  used  chiefly  in 
research.  For  class  work  the  author  uses  Miiller's  fluid  and  Miiller's 
fluid  and  formalin  almost  entirely.  Stains  are  apt  to  work  better 
after  chromic  fixing  fluids.  The  formulas  for  several  of  the  best 
fixing  agents  with  directions  for  their  use  are  found  in  the  last 
chapter. 

Washing. — Except  for  special  purposes,  fixing  fluids  are  washed 
out  of  the  tissues  in  running  water,  and  they  should  be  thoroughly 
removed.  For  this  purpose  the  author  has  made  a  galvanized 
iron  tank  in  which  a  gauze  tray  divided  into  small  gauze  compart- 
ments is  suspended.  The  water  is  brought  into  the  tank  through 
a  rubber  tube  with  the  mouth  resting  on  the  bottom,  and  leaves 
through  a  spout  at  the  top  to  which  another  tube  can  be  attached. 
In  this  way  a  large  number  of  specimens  can  be  washed  at  once 
and  their  identity  followed. 

Preserving  Tissues. — After  washing,  the  tissues  should  be  carried 
through   the  grades  of  alcohol,  and  may  be  preserved  for  a  con- 
siderable time  in  80  per  cent,  alcohol,  but  it  should  be  changed 
occasionally. 
(430/ 


GENERAL  HISTOLOGICAL  METHODS  431 

Choice  of  Sectioning  Methods. — The  choice  between  paraffin  and 
celloidin  for  embedding  depends  upon  the  character  of  the  section 
desired  and  the  nature  of  the  tissue.  Small  objects  and  those  of 
delicate  structure,  such  as  embryos,  dental  pulps,  etc.,  are  best 
sectioned  in  paraffin.  Large  pieces  and  blocks  containing  tissues 
of  different  densities  are  more  easily  cut  in  celloidin.  Paraffin 
can  be  cut  much  thinner  than  celloidin,  and  is  therefore  preferable 
for  the  minute  study  of  cell  structures  with  the  high  power.  Cel- 
loidin sections  are  more  easily  stained  and  are  easier  handled  and 
therefore  preferable  for  the  study  of  the  arrangement  of  tissues 
with  low  powers.  The  author  prefers  celloidin  sections  for  class 
work  whenever  possible. 

Embedding  in  Paraffin. — Tissues  fixed  and  washed  are  taken  from 
80  per  cent,  alcohol  and  placed  in  95  per  cent,  for  twenty-four 
hours,  then  in  absolute  alcohol  for  the  same  same  length  of  time, 
and  the  absolute  alcohol  should  be  changed  once  during  this  period, 
from  absolute  alcohol  to  xylol,  in  which  the  tissue  should  remain 
until  it  is  clear  and  translucent.  The  time  in  xylol  should  be  as 
short  as  possible,  as  it  has  a  hardening  action.  From  xylol  it  is 
placed  in  a  solution  of  paraffin  in  xylol,  and  from  this  to  soft  paraffin 
in  the  paraffin  oven,  at  a  temperature  of  not  over  52°  or  53°  C.  In 
this  it  should  remain  from  one-half  to  six  hours,  when  it  is  trans- 
ferred to  hard  paraffin  in  the  oven  for  the  same  length  of  time. 
The  time  in  the  oven  should  always  be  as  short  as  is  consistent 
with  a  perfect  infiltration.  After  sufficient  time  in  hard  paraffin 
the  tissue  is  blocked  in  the  following  way:  A  mold  is  made  by 
placing  L-shaped  pieces  of  metal  together  on  a  flat  slab.  These 
are  manufactured  for  the  purpose.  Melted  paraffin  is  poured  in 
the  mold  and  the  tissue  arranged  in  it,  placing  it  so  that  the  sec- 
tions will  cut  in  the  direction  desired.  A  film  of  paraffin  will  harden 
at  once  on  the  slab  and  the  tissue  can  be  placed  very  nicely  with 
the  needles.  As  soon  as  a  film  has  formed  over  the  surface  the 
slab  with  the  mould  should  be  immersed  in  cold  \vater,  so  as  to 
harden  the  paraffin  as  quickly  as  possible.  When  cold,  sections 
may  be  cut  at  once  or  the  block  may  be  preserved  in  a  pasteboard 
carton  properly  labelled.  As  a  rule,  paraffin  sections  should  be 
cut  as  soon  as  possible. 

Paraffin.— The  paraffin  for  embedding  tissues  must  be  of  the 
best  quality.  That  prepared  for  this  purpose  by  Griibler  is  prefer- 
able. It  should  be  of  two  grades,  that  melting  at  45°  C.,  and  that 
melting  at  54°  C.  The  hard  paraffin  is  mixed  with  the  softer, 


432  APPENDIX  CHAPTER  III 

so  as  to  give  a  melting-point  at  about  52°.  In  winter  softer  paraffin 
should  be  used  than  in  summer,  as  the  cutting  quality  depends 
upon  the  adjustment  of  the  paraffin  to  the  temperature  of  the  room. 
If  the  paraffin  is  too  hard  the  sections  are  liable  to  tear  and  curl; 
if  it  is  too  soft,  the  structure  of  the  tissue  will  be  disturbed  in 
cutting.  Perfect  infiltration  is  always  necessary  for  good  sections. 
Chloroform  or  oil  of  cedar  may  be  substituted  for  xylol  in  this 
process.  Xylol  is  most  rapid,  but  has  some  disadvantages  in  its 
action  on  the  tissues,  especially  if  left  too  long. 

Cutting  Paraffin  Sections. — If  the  specimen  has  been  placed  at 
one  end  of  the  block,  the  other  end  of  the  paraffin  may  be  clamped 
in  the  microtome.  If  the  piece  is  too  small,  it  should  be  fastened 
to  a  block  of  vulcanized  fiber  with  melted  paraffin  and  the  fiber 
block  clamped  in  the  specimen  holder.  With  a  sharp  scalpel  the 
excess  of  paraffin  around  the  specimen  should  be  trimmed  off, 
leaving  the  block  in  a  rectangular  form.  The  microtome  knife 
is  placed  at  right  angles  to  the  microtome  bed,  and  the  side  of  the 
block  should  be  parallel  with  the  blade.  The  specimen  should 
be  brought  up  just  to  the  edge  and  the  first  section  cut.  The  knife 
should  be  moved  with  a  quick,  sharp  motion,  as  paraffin  sections 
are  chopped  when  the  knife  is  in  this  position.  The  knife  is  pushed 
back,  the  block  lifted  with  the  micrometer  screw  so  as  to  give  a 
section  of  the  proper  thickness,  and  the  second  section  cut.  If 
the  paraffin  is  of  the  proper  consistency  and  the  block  has  been 
properly  trimmed,  the  edge  of  the  second  section  will  stick  to  the 
first  and  the  sections  stretch  out  over  the  knife  in  a  ribbon.  The 
ribbons  may  be  transferred  to  a  piece  of  clean  white  paper  and 
complete  series  of  sections  cut.  When  series  are  not  required  larger 
specimens  are  often  cut  better  by  placing  the  blade  of  the  knife 
obliquely  and  drawing  it  with  a  slow,  even  motion  through  the 
block.  If  the  sections  show  a  tendency  to  roll  up  when  the  corner 
of  the  section  begins  to  curl  over  the  edge  of  the  knife,  it  may  be 
caught  with  the  tip  of  a  camel's-hair  brush  and  so  section  after 
section  transferred  to  the  paper.  Paraffin  sections  should  cut  at 
a  thickness  of  from  seven  to  ten  microns,  but  sections  as  thin  as 
one  micron  may  be  cut  from  small  blocks  under  ideal  conditions. 

Handling  of  Paraffin  Sections. — For  staining,  paraffin  sections 
must  be  fastened  to  the  slide  or  cover-glass.  If  a  few  sections 
are  to  be  cut  the  slide  is  preferable;  if  many  sections,  as  in  the 
preparation  of  class  work,  square  cover-glasses  should  be  used. 
In  either  case  the  glass  must  be  absolutely  clean.  A  stock  of  per- 


GENERAL  HISTOLOGICAL  METHODS  433 

fectly  clean  slides  and  cover-glasses  should  always  be  kept  on  hand 
(see  p.  439).  A  thin  film  of  albumin  fixative  is  spread  upon  the 
glass;  this  film  must  be  as  thin  as  possible.  The  best  way  to  spread 
it  is  to  put  a  drop  of  fixative  on  a  glass  slab  or  an  ordinary  slide, 
touch  the  edge  of  the  drop  with  the  end  of  the  little  finger  and 
spread  it  over  the  cover-glass,  wiping  oft7  all  that  can  be  removed 
with  the  finger.  Lay  the  cover-glasses  film  side  up  on  a  piece  of 
paper  until  the  required  number  have  been  prepared.  As  each 
section  is  cut  it  is  laid  on  a  cover-glass,  straightened,  and  pressed 
down  with  a  camel's-hair  brush.  If  the  sections  curl  or  wrinkle 
they  should  be  floated  on  water  warmed  just  enough  to  soften  the 
paraffin  but  not  melt  it.  As  each  section  is  cut  it  should  be  dropped 
on  the  top  of  the  water,  where  it  will  straighten  out.  When  a 
number  have  been  placed  on  the  surface  of  the  water  they  may 
be  picked  up  by  holding  the  cover-glass  in  the  point  of  the  pliers 
and  slipping  it  underneath  the  section  and  lifting  it  as  on  a  section 


FIG.  343. — Morris  staining  dish. 

lifter.  The  water  is  drained  off  and  the  cover-glass  placed  in  the 
groove  of  the  tray  of  a  Morris  staining  dish,1  shown  in  Fig.  343. 
Each  tray  will  hold  about  thirty  cover-glasses.  They  must  now 
be  thoroughly  dried  by  leaving  them  over  night  at  room  tem- 
perature or  for  a  shorter  time  in  a  warm  oven,  which  should  not 
be  hot  enough  to  melt  the  paraffin.  When  dry,  each  cover-glass 
should  be  picked  up  in  the  pliers  and  passed  quickly  through  the 
middle  of  a  Bunsen  flame,  so  as  to  coagulate  the  albumin,  or  they 
may  all  be  fixed  at  once  in  an  oven.  Heat  that  will  just  melt  the 
paraffin  will  coagulate  the  albumin  and  hold  the  section  on  the 
glass.  By  means  of  a  little  wire  basket  the  tray  with  the  thirty 
cover-glasses  may  now  be  carried  from  one  dish  to  another  through 
the  following  necessary  reagents.  First,  a  minute  or  two  in  xylol 
to  remove  the  paraffin;  then  absolute  alcohol,  then  70  per  cent.; 
then  water;  Delafield's  hematoxylin  for  five  minutes;  distilled 
water  to  wash  off  the  stain;  acid  alcohol  (70  per  cent,  alcohol  to 
which  2  or  3  drops  of  hydrochloric  acid  has  been  added  to  every 

1  These  are  manufactured  by  Bausch  &  Lomb, 
28 


434  APPENDIX  CHAPTER  III 

100  c.c.  of  alcohol);  again  washed  in  tap  water  to  remove  and 
neutralize  the  acid  (some  prefer  alcohol  to  which  a  few  drops  of 
ammonia  have  been  added);  70  per  cent,  alcohol;  eosin  for  thirty 
seconds;  70  per  cent,  alcohol,  then  95  per  cent.,  then  absolute, 
and  finally  xylol.  From  the  xylol  the  sections  may  be  mounted 
or  given  out  to  the  class.  For  class  work  a  student  brings  to  the 
desk  a  clean  slide  with  a  drop  of  balsam  on  the  centre  and  receives 
a  section. 
Summary  of  Paraffin  Method. — 

Tissues  in  80  per  cent,  alcohol. 

95  per  cent,  alcohol,  twenty-four  hours. 

Absolute  alcohol  (changed  once),  twenty-four  hours. 

Xylol,  one-half  to  six  hours. 

Xylol  and  paraffin,  one-half  hour. 

Soft  paraffin,  one-half  to  six  hours. 

Hard  paraffin,  one  to  six  hours. 

Block. 

Section. 

Fix  on  glass. 

Heat. 

Xylol,  one  minute. 

Absolute  alcohol,  one  minute. 

95  per  cent,  alcohol,  same. 

70  per  cent,  alcohol,  same. 

Distilled  water. 

Hematoxylin,  five  to  ten  minutes 

Tap  water. 

Acid  alcohol. 

Tap  water  or  ammonia  alcohol. 

70  per  cent,  alcohol. 

Eosin,  thirty  seconds. 

70  per  cent,  alcohol. 

95  per  cent,  alcohol. 

Absolute  alcohol. 

Xylol. 

Mount  in  balsam. 

Label. 

Celloidin  Method. — Tissues  fixed  and  washed  are  taken  from  80 
per  cent,  alcohol  and  placed  in  95  per  cent,  for  twenty-four  hours; 
then  in  absolute  alcohol  for  the  same  length  of  time,  changing 
the  alcohol  once.  Then  into  a  mixture  of  absolute  alcohol  and 


GENERAL  HISTOLOGICAL  METHODS  435 

ether  for  twenty-four  hours,  from  this  into  a  thin  solution  of  cel- 
loidin,  in  which  they  should  remain  for  from  two  days  to  a  week. 
From  the  thin  solution  they  should  be  placed  in  a  thick  celloidin 
solution,  about  the  consistency  of  syrup,  for  the  same  length  of 
time.  The  tissues  may  be  kept  in  the  celloidin  solution  indefinitely 
without  injury,  and  if  the  tissue  is  difficult  to  infiltrate  it  may  be 
of  advantage  to  leave  them  in  these  solutions  for  weeks  or  months. 
In  this  case  the  bottles  must  of  course  be  perfectly  corked  to  prevent 
evaporation. 

Blocking  of  Celloidin  Material. — There  are  several  methods  for 
blocking  celloidin  materials,  of  which  the  author  prefers  the  fol- 
lowing: Thick  celloidin  is  poured  into  a  Stender  dish  or  a  small 
Petri  dish  until  there  is  enough  to  abundantly  cover  the  specimens, 
which  are  arranged  on  the  bottom  of  the  dish.  A  match  or  bit  of 
cork  is  placed  under  the  edge  of  the  cover  so  as  to  allow  slow 
evaporation.  In  a  day  or  two  the  celloidin  will  attain  the  consis- 
tence of  a  thick  jelly.  A  knife  is  now  passed  around  each  tissue 
and  the  celloidin  containing  the  specimen  lifted  out,  and  the  excess 
of  celloidin  is  trimmed  away.  A  vulcanized  fiber  block  has  one 
surface  dipped  into  the  thick  celloidin  and  the  specimen  arranged 
upon  it.  Thick  celloidin  is  now  added  to  surround  and  cover  the 
tissue  with  its  adherent  celloidin.  As  soon  as  this  is  hardened  so 
as  to  form  a  film  it  is  dropped  into  80  per  cent,  alcohol  to  harden 
the  entire  mass.  In  this  it  must  remain  at  least  twenty-four  hours 
before  it  can  be  sectioned.  Tissues  embedded  in  celloidin  may  be 
kept  for  years  in  80  per  cent,  alcohol  blocked  and  ready  to  cut 
without  great  injury  to  the  tissues. 

Celloidin  solutions  for  embedding  should  be  kept  in  two  grades 
and  labelled  "thick"  and  "thin"  celloidin.  The  latter  should  be 
quite  fluid,  the  former  about  a  syrup  consistence.  Scherring's 
celloidin  is  furnished  in  two  forms,  in  shreds  and  granules.  The 
former  will  dissolve  more  rapidly.  About  half  an  ounce  is  placed 
in  a  large-mouthed  bottle,  and  a  mixture  of  equal  parts  of  absolute 
alcohol  and  ether  added.  It  dissolves  slowly  and  should  be  shaken 
frequently.  When  this  solution  is  sufficiently  thick,  part  may  be 
poured  into  another  bottle  and  diluted  with  sufficient  absolute 
alcohol  and  ether  for  the  thin  solution,  while  the  thicker  portion 
is  poured  into  a  bottle  for  the  thick  solution,  and  absolute  alcohol 
and  ether  may  be  added  to  the  stock  bottle  to  dissolve  the  residue. 
When  blocking  tissues  as  described  above  the  trimmings  are  dropped 
back  into  the  stock  bottle. 


436  APPENDIX  CHAPTER  III 

Cutting  Celloidin  Sections. — The  fiber  block  is  clamped  in  the 
specimen  holder  and  adjusted.  The  knife  is  set  diagonally  so  as 
to  cut  with  a  drawing  motion,  and  both  the  knife  and  the  block 
are  kept  flooded  with  80  per  cent,  alcohol.  The  sections  may  be 
allowed  to  pile  up  on  the  knife,  and  after  eight  or  ten  are  cut  they 
are  slid  off  with  a  camel's-hair  brush  on  to  a  section  lifter  and 
transferred  to  80  per  cent,  alcohol,  in  which  they  may  be  kept  for 
some  time. 

Staining  Celloidin  Sections. — For  transferring  celloidin  sections 
the  most  convenient  thing  is  a  small  tea-strainer  with  a  handle. 
These  may  be  got  for  a  few  cents  at  any  hardware  store.  By  means 
of  this  the  sections  are  transferred  to  70  per  cent,  alcohol,  from  this 
to  distilled  water,  and  are  stained  from  five  to  ten  minutes  in  Dela- 
field's  hematoxylin.  The  stain  is  then  washed  off  with  tap  water, 
destained  with  acid  alcohol,  washed  in  tap  water  or  ammonia 
alcohol,  stained  thirty  seconds  in  eosin,  washed  with  70  per  cent, 
alcohol,  from  this  to  95  per  cent.,  in  which  they  should  be  given 
two  or  three  changes.  From  this  they  are  transferred  to  beech- 
wood  creosote  or  some  other  clearing  agent  (see  p.  445),  and  in  this 
they  may  be  kept  until  they  are  ready  to  mount  or  to  be  given  out 
to  the  class.  For  class  work  the  student  brings  to  the  desk  a  clean 
slide,  and  a  section  is  placed  upon  the  center  of  it.  After  blotting 
off  the  excess  of  oil  he  adds  a  drop  of  balsam,  covers  with  a  cover- 
glass,  and  labels  the  specimen. 
Summary  of  Celloidin  Method. — 

Tissues  in  80  per  cent,  alcohol. 

95  per  cent,  alcohol,  twenty-four  hours. 

Absolute  alcohol,  changed  twice,  twenty-four  hours. 

Absolute  alcohol  and  ether,  twenty-four  hours. 

Thin  celloidin,  two  days  to  a  week. 

Thick  celloidin,  the  same. 

Evaporate. 

Block. 

80  per  cent,  alcohol  to  harden  or  store. 

Sections  cut  in  80  per  cent,  alcohol. 

70  per  cent,  alcohol,  one  minute. 

Distilled  water. 

Hematoxylin,  five  to  ten  minutes. 

Tap  water. 

Acid  alcohol. 

Tap  water  or  ammonia  alcohol. 


GENERAL  HISTOLOGICAL  METHODS  437 

70  per  cent,  alcohol. 

Eosin,  one  miuute. 

70  per  cent,  alcohol  to  wash. 

95  per  cent,  alcohol,  changed  twice. 

Creosote. 

Mount  in  balsam. 

Label. 

Serial  Sections  with  Celloidin. — It  is  difficult  to  cut  series  of  sec- 
tions with  the  celloidin  method.  The  simplest  process,  and  one 
used  with  success,  is  to  carry  the  sections  in  order  from  the  knife 
to  the  slide,  arranging  three  or  four  at  one  end  of  it  and  leaving 
room  for  a  label.  Strips  of  porous  tissue  paper  are  cut  the  proper 
size  and  one  laid  over  the  sections  to  hold  them  in  place.  A  thread 
is  then  lightly  wrapped  around  the  slide  and  paper,  when  they  may 
be  carried  through  the  necessary  agents  for  staining,  in  Naples 
jars.  After  they  are  cleared  the  paper  is  removed,  the  excess  of 
the  oil  blotted  off,  the  balsam  put  upon  the  section  and  covered 
with  a  long  cover-glass. 

SPECIAL   METHODS. 

Dental  Pulp. — The  unerupted  premolars  from  a  young  sheep 
furnish  excellent  material  for  the  study  of  the  dental  pulp.  The 
jawrs  of  sheep  slaughtered  for  spring  lamb  can  be  easily  obtained 
from  the  stockyards,  and  while  still  warm  are  placed  in  Miiller's 
fluid  and  formalin,  in  which  they  are  taken  to  the  laboratory. 
The  temporary  incisors  are  still  in  place  and  may  be  used  for  peri- 
dental  membrane  material. 

With  the  bone  forceps  the  cortical  plate  is  removed  and  the 
unerupted  teeth  dissected  from  their  crypts.  By  grasping  the 
base  of  the  dental  papillae  with  the  pliers  the  pulp  may  be  pulled 
out  of  the  dentin.  They  should  then  be  replaced  in  Miiller's  fluid 
and  formalin  for  twenty-four  hours,  when  they  may  be  carried 
through  the  usual  process,  embedded  in  paraffin,  and  sectioned. 

Human  Pulps. — By  the  cooperation  of  the  extracting  room  human 
pulps  for  histological  work  may  be  obtained.  As  soon  as  extracted 
the  tooth  should  be  wrapped  in  a  gauze  napkin,  placed  in  the 
jaws  of  a  heavy  vise,  which  is  carefully  tightened  until  the  tooth 
cracks.  The  same  thing  may  be  accomplished  by  a  heavy  hammer 
on  an  anvil.  A  few  trials  of  this  will  enable  one  to  crack  the  tooth 
so  that  the  pulps  may  be  easily  removed  without  injury.  The 


438  APPENDIX  CHAPTER  III 

cracked  tooth  is  put  in  Miiller's  fluid  and  formalin  for  twenty- 
four  hours,  when  the  pieces  of  dentin  are  removed  and  the  pulp 
carefully  lifted  out  of  the  pulp  chamber.  It  is  then  carried  through 
the  regular  process,  embedded  in  paraffin,  and  sectioned.  If  the 
teeth  are  not  perfect  clinical  history  should  be  noted. 

Periosteum. — Young  kittens  that  have  not  attained  their  full 
growth  may  be  used  for  this  purpose.  The  bone  should  be  very 
carefully  dissected  so  as  not  to  injure  the  periosteum  and  then 
sawed  in  pieces,  using  a  fine  metal  saw.  It  is  usually  best  simply 
to  saw  it  in  two  at  the  middle  of  the  shaft  and  to  fix  it  in  Miiller's 
fluid  and  formalin.  After  fixing  and  washing,  it  should  be  cut  in 
small  pieces  and  decalcified  in  2  to  5  per  cent,  nitric  acid.  A  com- 
paratively large  volume  of  acid  should  be  used  and  a  pad  of  cotton 
placed  in  the  lower  half  of  the  bottle,  or  the  tissue  suspended  by  a 
thread.  It  is  best  to  change  the  acid  once  a  day.  Decalcifi cation 
may  require  from  two  days  to  a  week,  and  should  be  tested  by 
passing  sharp  needles  through  the  tissues.  As  soon  as  decalcified 
the  tissue  should  be  washed  for  twenty-four  hours  in  running 
water,  carried  through  the  grades  of  alcohol,  and  embedded  in 
celloidin.  The  sections  should  be  cut  at  right  angles  to  the  shaft. 

Peridental  Membrane. — For  class  work  the  peridental  membranes 
of  sheep  are  the  best  for  study,  as  their  fibers  are  large  and  their 
direction  easily  observed.  They  are  much  better  than  those  of 
either  cat  or  dog,  in  which  the  fibers  are  much  finer  and  the  bone 
more  dense.  The  jaws  are  brought  from  the  stockyards  in  Miiller's 
fluid  and  formalin,  the  crowns  broken  off  at  the  level  of  the  gum 
so  as  to  expose  the  pulp  chamber,  and  the  jaws  sawed  through  so 
as  to  leave  two  teeth  in  each  block,  after  which  they  are  replaced 
in  Miiller's  fluid  and  formalin  for  two  days,  decalcified  in  nitric 
acid,  and  thoroughly  washed.  They  may  now  be  cut  into  small 
blocks  for  transverse  sections  and  embedded  in  celloidin. 

Embryological  Material. — For  the  study  of  the  tooth  germ  in 
class  work  embryo  pigs  of  all  ages  are  easily  obtained.  The  entire 
embryo  should  be  at  once  placed  in  Miiller's  fluid  or  a  saturated 
solution  of  picric  acid  and  water.  In  Miiller's  fluid  they  should 
remain  a  week;  in  picric  acid,  forty-eight  hours.  After  fixing, 
the  heads  are  cut  off,  thoroughly  washed,  carried  through  the 
grades  of  alcohol,  and  embedded  in  paraffin. 


APPENDIX  CHAPTER  IV. 


FIXING   AGENTS   AND   STAINING   SOLUTIONS. 

Cleaning  of  Slides  and  Cover-glasses. — Slides  or  cover-glasses  on 
which  paraffin  sections  are  to  be  mounted  must  be  absolutely 
clean.  They  should  be  dropped  in  strong  sulphuric  acid  and  allowed 
to  remain  a  few  minutes.  The  acid  should  then  be  poured  off  and 
thoroughly  removed  with  water,  and  strong  acetic  acid  poured  on. 
After  remaining  a  few  minutes  wash  the  acid  off  thoroughly  and 
wipe  from  alcohol.  Keep  ready  for  use  in  a  clean  box. 

Meyer's  Fixative. — The  white  of  an  egg  is  chopped  with  a  pair 
of  scissors  and  filtered  through  muslin,  diluted  with  an  equal  volume 
of  glycerin,  and  a  little  sodium  oxalate  added  to  prevent  decom- 
position. 

FIXING  AGENTS. 

Flemming's  Solution. — A  good  solution  for  fixing  nuclear  struct- 
ures is  the  chromic  acid  solution  of  Flemming: 

Parts. 

Osmic  acid,  1  per  cent,  aqueous  solution 10 

Chromic  acid,  1  per  cent,  aqueous  solution 25 

Glacial  acetic  acid,  1  per  cent,  aqueous  solution 10 

Distilled  water 55 

Small  pieces  are  fixed  in  a  small  quantity  of  the  fluid  for  at  least 
twenty-four  hours.  They  are  then  washed  for  the  same  number 
of  hours  in  running  water  and  passed  through  50,  75,  and  80  per 
cent,  each  twenty-four  hours  into  90  per  cent,  alcohol. 

A  stronger  solution  is  made  as  follows: 

Parts 

Osmic  acid,  2  per  cent,  aqueous  solution 4 

Chromic  acid,  1  per  cent,  aqueous  solution 15 

Glacial  acetic  acid 1 

Fol's  Solution. — A  modification  of  Flemming's  solution. 

Parti 

Osmic  acid,  1  per  cent,  aqueous  solution 2 

Chromic  acid,  1  per  cent,  aqueous  solution 25 

Glacial  acetic  acid,  2  per  cent,  aqueous  solution 5 

Distilled  water 68 

(439) 


440  APPENDIX  CHAPTER  IV 

Corrosive  Sublimate. — An  excellent  fixing  fluid  is  made  by  satu- 
rating distilled  water  with  corrosive  sublimate.  Small  pieces 
about  0.5  cm.  in  diameter  are  immersed  in  this  fluid  for  from  three 
to  twenty-four  hours,  then  washed  in  running  water  for  twenty- 
four  hours,  and  then  transferred  into  70  per  cent,  alcohol.  After 
twenty-four  hours  the  tissues  are  placed  in  80  per  cent,  for  the  same 
length  of  time  and  then  preserved  in  90  per  cent.  It  often  occurs 
that  after  changes  in  temperature  crystals  of  sublimate  are  formed 
on  the  surface  or  in  the  interior  of  the  object.  For  their  removal 
a  few  drops  of  iodine  and  potassium  iodide  are  added  to  the  alcohol 
(P.  Mayer).  It  is  a  matter  of  indifference  whether  the  70  per  cent., 
80  per  cent.,  or  90  per  cent,  alcohol  is  thus  iodized.  In  future 
treatment  of  the  object,  as  well  as  in  sectioning,  any  such  crystals 
of  sublimate  will  not  be  found  to  be  a  hindrance.  In  the  case  of 
delicate  objects  it  is  better  to  undertake  their  removal  after  sec- 
tioning by  adding  iodine  to  the  absolute  alcohol  then  used. 

Acetic  Sublimate  Solution. — An  excellent  solution  specially  used 
for  embryonic  tissues  and  for  organs  containing  only  a  small  quan- 
tity of  connective  tissue.  To  a  saturated  aqueous  solution  of  sub- 
limate, 5  to  10  per  cent,  of  glacial  acetic  acid  is  added.  After 
remaining  two  to  three  hours  or  more  in  this  solution,  the  objects 
are  transferred  to  35  per  cent,  alcohol  and  then  passed  through 
the  higher  grades  of  alcohol. 

Picric  Acid.— Small  and  medium-sized  objects  (up  to  1  c.c.)  are 
fixed  in  twenty-four  hours  in  a  saturated  aqueous  solution  of  picric 
acid  (about  0.75  per  cent.).  Objects  of  considerable  size  may  be 
left  in  this  solution  for  weeks  without  detriment.  The  tissues 
are  then  transferred  to  70  or  80  per  cent,  alcohol,  in  which  they 
remain  until  the  alcohol  is  not  colored  by  the  picric  acid.  Instead 
of  a  pure  solution  of  picric  acid,  the  picrosulphuric  acid  of  Kleinen- 
berg,  or  the  picric  acid  of  P.  Mayer  may  be  used.  Picrosulphuric 
acid  is  made  as  follows:  1  c.c.  of  concentrated  sulphuric  acid  is 
added  to  100  c.c.  of  a .  saturated  aqueous  picric  acid  solution. 
Allow  this  to  stand  for  twenty-four  hours  and  dilute  with  double 
its  volume  of  distilled  water.  The  picric  acid  solution  is  made  by 
adding  2  c.c.  of  pure  nitric  acid  to  100  c.c.  of  saturated  picric  acid 
solution.  Filter  after  standing  for  twenty-four  hours. 

Chromic  Acid. — Chromic  acid  is  used  in  a  3  to  1  per  cent,  aqueous 
solution.  Small  pieces  are  fixed  for  twenty-four  hours,  larger  ones 
for  a  longer  time.  The  quantity  of  the  fixing  fluid  should  equal  at 
least  more  than  fifty  times  the  volume  of  the  tissues  to  be  fixed. 


FIXING  AGENTS  441 

After  fixing,  objects  must  be  washed  for  at  least  twenty-four  hours 
in  running  water,  then  through  the  grades  of  alcohols,  and  preserved 
in  80  per  cent.    Two  to  3  drops  of  formic  acid  to  every  100  c.c.  of 
chromic  acid  solution  improve  their  fixing  properties. 
MiiUer's  Fluid.— 

Potassium  bichromate 2  to  2.5  grams 

Sodium  sulphate 1      gram 

Water 100      c.c. 

This  solution  requires  a  long  time  for  fixing,  at  least  several 
weeks,  and  for  large  pieces  several  months.  During  the  first  few 
weeks  the  solution  should  be  changed  every  three  or  four  days 
and  later  once  a  week,  until  it  remains  clear.  Tissues  should  be 
thoroughly  washed  in  running  water  at  least  twenty-four  hours. 
For  some  special  purposes  it  is  better  to  wash  in  alcohol.  Tissues 
should  be  carried  through  the  grades  and  preserved  in  80  per  cent, 
alcohol.  While  tissues  are  in  MiiUer's  fluid  they  should  be  kept  in 
the  dark. 

Miiller's  Fluid  and  Formalin. — 

Muller's  fluid 100  c.c. 

Formalin 10  c.c. 

The  addition  of  formalin  to  Muller's  fluid  greatly  hastens  fixa- 
tion. It  is  an  excellent  agent  of  great  penetrating  power,  and  tissues 
stain  very  well  after  it.  Twenty-four  hours  will  fix  tissues  of  ordi- 
nary size,  though  they  may  be  left  longer  without  damage.  Bone 
fixed  too  long  in  formalin  is  liable  to  be  hard  to  cut. 

Zenker's  Fluid. — 

Grams. 

Potassium  bichromate 2.5 

Sodium  sulphate 1.0 

Corrosive  sublimate 5.0 

Glacial  acetic  acid 5.0 

Water 100.0 

Add  the  glacial  acid  in  proper  proportion  to  the  quantity  of  the 
solution  to  be  used,  and  not  to  the  stock  solution.  Allow  the  tissues 
to  remain  in  this  solution  for  from  six  to  twenty-four  hours.  Then 
wash  in  running  water  for  from  twelve  to  twenty-four  hours  and 
transfer  to  gradually  concentrated  alcohol.  Crystals  of  sublimate 
which  may  be  present  are  removed  with  iodized  alcohol.  Zenker's 
fluid  penetrates  easily  and  fixes  nuclear  and  protoplasmic  structures 
equally  well  without  decreasing  the  staining  qualities  of  the  elements. 


442  APPENDIX  CHAPTER  IV 

Formalin. — Of  recent  years  formalin,  which  is  a  4  per  cent,  solu- 
tion of  the  gas  formaldehyde  in  water,  has  been  much  used  as  a 
fixing  fluid.  Make  a  solution  by  adding  10  parts  of  formalin  to  90 
parts  of  water  or  normal  saline  solution.  Small  pieces  of  tissue 
should  remain  in  this  for  from  twelve  to  twenty-four  hours,  larger 
pieces  a  number  of  days  or  weeks,  and  then  transfer  to  90  per  cent, 
alcohol. 

STAINING   AGENTS. 
Delafield's  Hematoxylin. — 

Hematoxylin  crystals 4  grams 

Absolute  alcohol 25  c.c. 

Ammonia  alum,  aqueous  solution 400  c.c. 

Methyl  alcohol 100  c.c. 

Glycerin 100  c.c. 

Dissolve  hematoxylin  crystals  in  absolute  alcohol  and  add  to 
the  alum  solution,  place  in  an  open  vessel  for  four  days,  then  filter 
and  add  the  methyl  alcohol  and  glycerin. 

Hemalum  (Mayer,  91).- — One  gram  of  hematin  is  dissolved  by 
heating  in  50  c.c.  of  absolute  alcohol.  This  is  poured  into  a  solu- 
tion of  50  grams^of  alum  in  1  liter  of  distilled  water  and  the  whole 
well  stirred.  A  thymol  crystal  is  added  to  prevent  the  growth  of 
fungus/lyThe  advantages  of  hemalum  is  as  follows:  The  stain  may 
be  used  immediately  after  its  preparation,  it  stains  quickly,  never 
overstating,  especially  when  diluted  with  water,  and  penetrates 
deeply,  making  it  useful  for  staining  in  bulk.  After  staining  sec- 
tions or  tissues  are  washed  in  distilled  water. 

Safranin. — 

Safranin 1  gram 

Absolute  alcohol 10  c.c. 

Aniline  water 90  c.c. 

Aniline  water  is  prepared  by  shaking  up  5  c.c.  to  8  c.c.  of  aniline 
oil  in  100  c.c.  of  distilled  water  and  filtered  through  a  wet  filter. 
Dissolve  the  safranin  in  the  aniline  water  and  add  the  alcohol. 
Filter  before  using. 

Stain  sections  fixed  in  Flemming's  solution  for  twenty-four 
hours  and  decolorize  with  a  weak  solution  of  hydrochloric  acid  in 
absolute  alcohol  (1  to  1000).  After  a  varying  period  of  time, 
usually  only  a  few  minutes,  all  the  tissue  elements  will  be  found 
to  have  become  bleached,  only  the  chromatin  of  the  nucleus  retain- 
ing the  color. 


STAINING  AGENTS  443 

Methyl  Green. — Stains  very  quickly.  One  gram  is  dissolved  in 
100  c.c.  of  distilled  water  to  which  25  c.c.  of  absolute  alcohol  is 
added.  Rinse  the  sections  in  water,  then  place  in  70  per  cent, 
alcohol  for  a  few  minutes,  transfer  to  absolute  alcohol  for  a  minute, 
etc. 

Hematoxylin. — Van  Gieson's  Acid  Fuchsin-Picric  Acid  Solution. — 
Stain  in  any  of  the  hematoxylin  solutions,  and  after  rinsing  sec- 
tions in  water  counter-stain  in  the  following: 

Acid  fuchsin,  1  per  cent,  aqueous  solution 5  c.c. 

Picric  acid,  saturated  aqueous  solution 100  c.c. 

Dilute  with  an  equal  quantity  of  water  before  using.  The  hema- 
toxylin stained  sections  remain  in  the  solution  from  one  to  two 
minutes,  are  then  rinsed  in  water,  dehydrated,  and  cleared. 

Hematoxylin-Eosin. — Sections  already  stained  in  hematoxylin 
are  placed  for  two  to  five  minutes  in  a  1  to  2  per  cent,  aqueous 
solution  of  eosin  or  in  a  1  per  cent,  solution  of  eosin  in  a  60  per 
cent,  solution  of  alcohol.  They  are  then  washed  in  water  until 
free  from  the  stain,  after  which  they  remain  for  a  short  time  in 
absolute  alcohol.  In  place  of  the  eosin  solution  a  1  per  cent,  aqueous 
solution  of  benzopurpurin  may  be  used  for  the  following  solution 
of  erythrosin  (Held). 

Erythrosin 1  gram 

Distilled  water 150  c.c. 

Glacial  acetic  acid 3  drops 

Silver  Nitrate  Method. — Especially  useful  for  staining  intercellular 
substances  of  epithelium,  endothelium,  and  mesothelium,  and 
the  ground  substance  of  connective  tissues.  It  may  be  used  on 
either  fresh  or  fixed  tissues,  fresh  tissue,  however,  being  more 
satisfactory.  Spread  the  tissues  to  be  stained  in  thin  layers; 
immerse  in  a  0.5  to  1  per  cent,  solution  of  silver  nitrate  from  ten 
to  fifteen  minutes;  rinse  in  distilled  water  and  place  in  fresh  dis- 
tilled water  or  70  per  cent,  alcohol  or  a  4  per  cent,  solution  of  for- 
malin and  expose  to  direct  sunlight  until  they  assume  a  brown  color. 
The  sunlight  reduces  the  silver  in  the  form  of  fine  particles  which 
appear  black  on  being  examined  with  transmitted  light.  The 
preparations  thus  obtained  may  be  examined  in  glycerin  or  dehy- 
drated and  mounted  in  balsam. 

Glycerin. — To  mount  in  glycerin  transfer  the  sections  from  water 
to  the  slide,  cover  with  a  drop  of  glycerin,  and  apply  the  cover-slip. 


444  APPENDIX  CHAPTER  IV 

Sections  colored  with  a  stain  that  would  be  injured  by  contact 
with  alcohol  and  where  clearing  is  not  especially  necessary  are 
mounted  this  way. 

Farrant's  Gum  Glycerin. — In  place  of  pure  glycerin  the  following 
mixture  may  be  used : 

Glycerin 50  c.c. 

Water 50  c.c. 

Gum  arabic  (powder) 50  grams 

Arsenous  acid 1  gram 

Dissolve  the  arsenous  acid  in  water.  Place  the  gum  arabic  in  a 
glass  mortar  and  mix  it  with  the  water,  then  add  the  glycerin. 
Filter  through  a  wet  filter  paper  or  through  fine  muslin.  To  pre- 
serve such  preparations  for  any  length  of  time  the  cover-glasses 
must  be  so  fixed  as  to  shut  off  the  glycerin  from  the  air.  For  this 
purpose  cements  or  varnishes  are  used,  by  painting  over  the  edges 
of  the  cover-glass.  These  masses  adhere  to  the  glass,  harden,  and 
fasten  the  cover-glass  firmly  to  the  slide,  hermetically  sealing  the 
object.  Kronig's  is  one  of  the  best  formulas  for  varnish,  and  is 
made  as  follows:  Melt  2  parts  of  wax  and  stir  in  7  to  9  parts  of 
colophpnium  and  filter  the  mass  hot.  Before  employing  an  oil 
immersion  lens  it  is  best  to  paint  the  edges  with  an  alcoholic  solution 
of  shellac. 

Silver  Nitrate. — In  thin  membranes  and  sections  the  vessel  walls 
can  be  rendered  distinct  by  silver  impregnation,  which  brings  out 
the  outlines  of  their  endothelial  cells.  This  may  be  done  either 
by  injecting  the  vessel  with  a  1  per  cent,  solution  of  silver  nitrate, 
or  with  a  0.25  per  cent,  solution  of  silver  nitrate  in  gelatin.  This 
method  is  of  advantage,  since  after  hardening  the  capillaries  of 
the  injected  tissues  appear  slightly  distended.  Organs  thus  treated 
can  be  sectioned,  but  the  endothelial  mosaic  of  the  vessels  does 
not  appear  definitely  until  the  sections  have  been  exposed  to  sun- 
light. 

The  injections  of  lymph  channels,  lymph  vessels,  and  lymph 
spaces  is  usually  done  by  puncture.  A  pointed  cannula  is  thrust 
into  the  tissue  and  the  syringe  empties  by  a  slight  but  constant 
pressure.  The  injected  fluid  spreads  by  means  of  the  channels 
offering  the  least  resistance.  For  this  purpose  it  is  best  to  use 
aqueous  solution  of  Berlin  blue  or  silver  nitrate,  as  the  thicker 
gelatin  solutions  cause  tearing  of  the  tissues. 


STAINING  AGENTS  445 

Clearing  Agents. — Clearing  agents  are  substances  of  high  refract- 
ing index,  mostly  oils,  which  are  used  to  displace  alcohol  and  pre- 
pare tissues  for  embedding  and  sections  for  mounting  in  balsam. 

Clearing  agents  for  embedding  in  paraffin  must  be  miscible  with 
alcohol  and  solvents  for  paraffin.  They  are  called  clearing  agents 
because  the  tissues  become  translucent  and  clear  in  them.  Xylol 
is  the  most  rapid  and  probably  most  used  agent.  It  has,  however, 
a  hardening  action  on  the  tissues,  especially  if  they  remain  too 
long  in  it.  Pure  oil  of  cedarwood  when  free  from  turpentine  is 
an  excellent  agent.  Chloroform  has  been  largely  used  for  the  same 
purpose. 

Before  celloidin  sections  are  mounted  in  balsam  they  must  be 
cleared.  For  this  purpose  an  oil  that  will  mix  with  95  per  cent, 
alcohol  is  desirable,  as  absolute  alcohol  softens  the  celloidin.  The 
oil  used  must  not  dissolve  the  celloidin,  and  should  not  dissolve 
the  stain.  Beechwood  creosote  is  an  excellent  agent,  and  has  been 
largely  used.  It  clears  sections  rapidly  from  95  per  cent,  alcohol. 
Oil  of  bergamot  is  an  excellent  agent,  also  oil  of  origanum;  but  in 
the  latter  the  oleum  origani  cretici  and  not  the  oleum  origani 
gallici  must  be  used.  A  mixture  of  equal  parts  of  oil  of  bergamot 
and  beechwood  creosote  has  been  used  satisfactorily,  and  is  an 
excellent  agent.  A  cheaper  mixture  is  made  of  equal  parts  of 
phenol,  oil  of  origanum,  and  oil  of  cedarwood. 


INDEX. 


ABSCESS,  absorption  of  roots  before  and 

after,  284 
Absorbent  organ,  282 

osteoclasts  as,  287 
Absorption  of  bone,  276 
of  cementum,  163 
of  deciduous  tooth  roots,  279 

causes  of,  275 
of  dentin,  280 
of  enamel,  280 
of  implanted  teeth,  282 
of  permanent  tooth  roots,  284 
Acetic  acid  and  sublimate  for  fixing,  440 
Acrodont  teeth,  235 
Alveolar  bone,  338 

relation  of,  to  mandible,  27 

to  teeth,  26 

removal  of,  physiologically,  27 
crest  group  of  fibers,  241 
division  of  peridental  membrane, 

239 

Alzheimer,  279 
Ameloblasts,  327 
Amphioxus,  19 
Anolpgies,  definition  of,  23 

illustration  of,  23 
Antrum  of  Highmore.     See  Maxillary 

sinus. 
Apical  division  of  peridental  membrane, 

239 

group  of  fibers,  241 
Arey,  276 
Aristotle,  183 
Attachment  of  teeth,  230 
by  ankylosis,  233 
fibrous  membrane,  231 
hinge  joint,  231 
insertion  in  a  socket,  235 


B 


BALSAM  in  grinding,  417 
Bibra,  von,  on  enamel,  38 
Black  on  absorption  of  roots,  286 

on  epithelial  cords,  260 

on  periosteum,  222 
Bland-Sutton  on  absorptions,  277 


Blastoderm  formation,  310 

layers  of,  308 
Blastula,  307 
Blocking  of  celloidin  material,  434 

of  paraffin  material,  431 
Blood  supply  of  ameloblasts,  283 
of  osteoclasts,  280 
of  pulp,  171 
Bloodvessels  in  cementum,  153 

of  peridental  membrane,  267 

of  pulp,  171 
Bohm,  277 
Bone,  209 

and  cementum  compared,  393 

arrangement  of,  213 

canaliculi,  211 

cancellous,  336 

compact,  213 

construction    and    destruction    in 
bone  building,  214 

Cope  on,  340 

corpuscles  of,  211 

decalcified,  393 

definition  of,  209 

distribution  in  mandible,  341 

endochondral,  216 

endomembranous,  218 

fibers  of  Sharpey  in,  160 

formation  and  growth  of,  216 

ground  sections  of,  392 

growth  of,  219 

Haver  sian  system,  211 

influences  of  mechanical  forces  on, 
350 

interstitial,  214 

lacunae  of,  211 

compared    with     lacunae     of 
cementum,  158 

matrix  of,  210 

osteoclasts  in,  214,  217 

periosteal  buds  in,  217 

relation  of  teeth  to,  26,  334 

structural  elements  of,  209 

subperiosteal,  211 

compared  with  cementum,  394 

varieties  of,  211 

Volkman's  canals  in,  211 
Branchial  arches,  313 

arteries,  313 

clefts,  313 

(447) 


448 


INDEX 


Branching  of  dentinal  tubules  in  crown, 

139 

in  root,  143 
Bredichin,  276 
Brooks,  Dr.,  201 
Burchard  and  Inglis,  285 


CALCIFICATION,  beginning  of,  of  teeth, 
325 

of  bone,  216 

of  cememtum,  153 

of  dentin,  326 

of  enamel,  326 

Calcium  carbonate  in  enamel  and  den- 
tin,  38,  136 

fluoride  in  enamel  and  dentin,  38, 
136 

phosphate  in  enamel  and  dentin, 

38,  136 
Canaliculi  of  bone,  211 

of  cementum,  158 
Caries  of  dentin,  59 

of  enamel,  48 

granular  layer  of  Tomes  and,  146 

intensity  and  liability  of,  60 

interglobular  spaces  and,  149 

secondary  or  backward,  62 

stages  in  progress  of,  56. 
Carotid,  internal,  189 
Cartilage  in  bone  formation,  216 

in  enamel  analysis,  38 

Meckel's,  325 
Causch,  277 
Cavities,  classes  of,  96 

relation  of,  to  marginal  ridges,  127 

rod  direction  in  preparation  of,  97 

structural  requirements  of,  90 
Cavo-surface  angle,  90 

requirements   in    preparation 

of,  90 
Cell  division,  300 

theory  of,  229 

walls,  202 

Celloidin,  blocking  of,  434 
method  of,  437 

cutting  of,  436 

serial  sections  of,  437 

staining  of,  436 

stock  solutions  of,  435 
Cementing  substance,  43 
Cement oblasts,  251 
Cementum,  153 

absorption  of,  163,  278 

compared  with  bone,  293 

corpuscles  of,  160 

definition  of,  153 

development  of,  153 

distribution  of,  32 


Cementum,  function  of,  153 
Haversian  canals  in,  153 
histogenesis  of,  154 
imbedded  fibers  in,  160 
lacunae  of,  158 

compared  with  those  of  bone, 

158 

lamellae  of,  154 
structural  elements  of,  154 
Cervical  division  of  peridental  mem- 
brane, 239 
Chemical  composition  of  dentin,  136 

of  enamel,  38 
ideas,  301 

Chromic  acid  for  fixing,  440 
Chromosomes,  vehicles  of  transmission, 

301 

Chronology  of  dental  follicle,  329 
Cleaning  slides,  439 
Clearing  agents,  445 
Cleft  palate,  319 
Compensating  canals,  284 
Connective  tissue,  203 

cells  becoming  phagocytic,  279 
chemical  relation   of  formed 
materials  to  cytoplasm,  207 
mutations  of,  203 
response  to  chemical  changes, 

207 

Cope  on  bone,  340 
Corrosive  sublimate  for  fixing,  440 
Cortical  plates,  336 
Creosote,  445 
Cribriform  plates,  331 
Cryer,  338 
Czermak  on  interglobular  spaces,  146 


D 

DAUTSCHAKOFF,  276 
Decalcification  of  bone,  438 

by  osteoblasts,  278 

in  osteomalacia,  276 

of  teeth,  438 

Delafield  and  Pruden,  277 
Dental  caries,  48 

of  dentin,  59 
of  enamel,  48 

follicle,  324,  331 

lamina,  331 

ligament,  342 

papilla,  323 

pulp,  164 

ridge,  321,  331 
Dentin,  135 

absorptions  of,  281 

calcification  of,  326 

caries  of,  59 

changes  with  aye,  137 

chemical  analysis  of,  136 


INDEX 


449 


Dentin,  clear  layer  of,  145 
defects  in,  146 
definition  of,  135 
development  of,  326 
distribution  of,  29 
fibrils  of,  144 

formative  cells  of,  113,  166 
function  of,  135 
granular  layer  of,  145 
histogenesis  of,  135 
interglobular  spaces  in,  146 
lines  of  Schreger,  149 
matrix  of,  136 
secondary,  150 
sheaths  of  Newman,  137 
structural  elements  of,  135 
tubules  of,  138 

branching  of,  143 
caries  in,  61 
curves  of,  139 
diameter  of,  138 
direction  of,  in  crown,  139 

in  root,  143 

Dento-cemental  junction,  144 
Dento-enamel  junction,  31,  139 
and  caries,  62 
characteristics  of,  143 
sensitiveness  of,  143 
Dermal  scales,  22,  230 
Descriptive  terms,  34 
Dewey,  Dr.  Kaethe,  175,  197 
Dissecting,  426 

Dog  teeth,  absorptions  of,  279 
Duval,  277 


ECTODERM.     See  Epiblast. 
Embryology,  302 

biological    considerations    funda- 
mental, 298 

chemical  ideas  related  to,  301 

earlv  stages  of,  302 

of  teeth,  321 
Enamel,  28,  37 

abrasion  of,  96 

absorption  of,  280 

action  of  acid  on,  45 

appearances  of,  67 

areas  of  weakness,  125 

bands  of  Retzius,  70 

blood  supply   of  formative   cells, 
116,  284 

calcification  of,  326 

caps,  113 

cavity  walls  in,  90 

cementing  substances  of,  43 

characteristics  of,  63 

chemical  composition  of,  38 

cleavage  of,  84 
29 


Enamel  cuticle,  74 
defects  in,  113 
degree  of  calcification  of,  38 
development  of,  326 
differences  between,  and  other  cal- 
cified tissue,  37 
rods  and  cementing  sub- 
stance, 43 

direction  of  rods,  41,  78 
distribution  of,  28 
effect  of  caries  on,  48 
of  elephant's  tusk,  29 
etching  of,  45 
function  of,  28 
gnarled,  64 
of  herbivora  teeth,  34 
histogenesis  of,  37 
hypoplasia  of,  82 
incremental  lines  of,  70 
lines  of  Schreger  in,  73 
Nasmyth's  membrane,  74 
organ,  322 

ameloblasts  of,  327 

blood  supply  of,  283 

development  of,  322 

effect  on  mesenchymal  tissue, 
322 

loss  of,  40 

of  molar  teeth,  permanent,  327 

remains  of,  261 

stellate  reticulum  of,  323 

stratum  intermedium  of,  332 

tunics  of,  323 
origin  of,  37 
planing  of,  87 
refraction  of,  43 

relation  of  formative  organ  to,  40 
relative  solubility  of,  44 

strength  of  rods  and  cement 

substance,  43 
of  rodent  teeth,  34 
rods,  41 

diameter  of,  41 

direction  of,  41,  78 

length  of,  42 

refraction  of,  43 

size  of,  41 
spindles,  76 
straight,  64 
stratification  of,  68 
striation  of,  67 
structural  defects  of,  113 

elements  of,  37 
Tomes  on,  39 
Williams  on,  39 
Endoskeleton,  19 
Endothelial  cells  as  phagocytes,  277 

Mallory  on, .277 
relation  to  nervous  system,  22 
Epiblast,  308 
Epithelial  cords,  260 


450 


INDEX 


Epithelial  cords,  arrangement  of,  261 

of  cells  in,  263 
Black  on,  260 
derivation  of,  261 
distribution  of,  261 
as  lymphatics,  260 
von  Brunn  on,  260 

Eruption  of  teeth,  275 

Etching  of  enamel,  45 

Eustachius,  183 

Exoskeleton,  19 


FACIAL  artery,  197 

Farrant's  gum  glycerin,  444 

Fastening  teeth  to  disks,  413 

Fat  in  dentin,  136 
in  enamel,  38 

Fertilization,  304 

Fibers  of  peridental  membrane,  240 
classification  of,  241 
imbedded     in      alveolar 

process,  241 
in  cementum,  160 

Fibrils  of  odontoblasts,  144,  168 

Fibroblasts  in  peridental  membrane, 
250 

Filiform  papillae,  293 

Fischer,  277 

Fixatives,  427,  430 

Fixing,  426 

Flemming's  solution,  439 

Follicle,  dental,  324,  331 

Fol's  solution,  439 

Foramen,  apical,  171 

Forces  influencing  bone  growths,  340, 
349 

Frontal  nasal  process,  318 

Fungiform  papillae,  293 


G 

GASTRULA,  307 
Germ  layers,  308 
Giant  cells,  277 
Gilmer,  282 
GingivgD,  244 

lymph  vessels  of,  195 
Gingival  division  of  root,  238 
group  of  fibers,  241 
space,  epithelium  of,  264 
Glands  of  Serres,  264 

of  tongue,  290 
Glycerin  for  mounting,  443 
Gomphosis,  236 
Granular  layer  of  Tomes,  145 

difficulty  of  staining,  145 
invisibility  of,  in  haema- 
toxylin       and       eosin 
stain,  145 


Granular  layer  of  Tomes,  Skillen's  stain 

for,  145 

Grinding  of  crumbled  material,  420 
disks,  408 

in  hard  balsam,  416 
of  frail  material,  416 
machine,  403 

clogging  of  stones  of,  421 
fastening  teeth  to  disks  of,  413 
lap  wheels  for,  409 
point  finder  of,  409 
preparation  of  shellac  for,  419 
slicing  mechanism  for,  422 
spatter  guards  for,  411 
spiders  and  dogs  for,  412 
stones  for,  410 
watering  stones  of,  410 
rapidity  of,  414 
removal  of  cover-glass  from  disk 

of,  418 
of  tooth  sections,  379 

process  of,  410 
Growth  force,  349 
Gubernaculum  dentis,  275 
Gum,  289 

epithelium  of,  288 
fibers  of,  289 


H^MALTJM,  442 

Haematoxylin  and  eosin,  443 

Delafield's,  442 

failure  to  stain  granular  layer  of 

Tomes,  145 

Hair  compared  with  tooth,  24 
Hardening,  427 
Hare  lip,  320 
Hassin,  279 
Haversian  systems  of  bone,  211 

of  cementum,  153 
Hertwig's  embryology,  230,  320 
Hess,  284 
Hinged  teeth,  231 
Histological  technic,  424 
Holoblastic  segmentation,  306 
Homology,  19,  23 
Horizontal  group  of  fibers,  241 
Howell,  276 
Howship's  lacunas,  255 
Huber  on  pulpal  nerves,  177 
Huxley,  75 
Hypoblast,  308 
Hypoplasia  of  enamel,  82 


IMPLANTED  teeth,  absorbed,  282 
Incremental  lines,  70 


INDEX 


451 


Indexing  and  filing,  427 
Inferior  dental  nerve,  354 
Inglis,  285 
Intercellular  substances,  200 

in  pulp,  171 

kinds  of,  202 
relation  of  cells  to,  201 
Interglobular  spaces,  146 

Czermak  on,  146 
Intermaxillary  bone,  319 


JACKSON,  276 

Jaws,  changes  with  age,  26 

growth  of,  347 
Jugular,  internal,  189 


KERATINIZED  scales,  19 
Kolliker  on  osteoclasts,  276 
Krause,  183 


LABELLING  of  slides,  429 
Laboratory  methods,  430 

directions  for  students,  381 
Lacunse  of  bone,  211 
of  cementum,  158 

compared  with  bone,  393 
Lamellae  of  cementum,  154 
Lansit,  19 
Lap-wheels,  409 
Lateral  nasal  process,  318 
Layer  of  Weil,  171 
Leukocytes  in  lymph  stream,  183 

as  origin  of  osteoclasts,  277 
Ligamentum  circulare,  dental  ligament, 

242 
penetrated  by  lymph  channels, 

195 

Lingual  tonsils,  297 
Lymphatics,  central  trunks  of,  199 
character  of  fluid  of,  183 
coagulation  of  fluid  of,  181 
collecting  trunks  of,  184,  186 
descending  cervical  chain  of,  190 
Dewey's  work  on,  197 
discovery  of,  183 
Eustachius,  183 
external  glands  of,  190 
function  of,  181 
of  gingivae,  195 
of  head  and  neck,  186 

mastoid  group,  188 
parotid    and    subparotid 
group,  189 


Lymphatics  of  head  and  neck,  retro- 
pharyngeal  group,  190 
submaxillary  group,  189 
submental  group,  190 
suboccipital  group,  188 
internal  glands,  190 
of  lips,  192 

lymphatic  duct  (right),  182 
Massa  on,  183 
of  mouth  and  gums,  193 

inner    surface    of    man- 
dible, 193 
of  maxillae,  193 
outer    surface    of    man- 
dible, 193 
of  maxillae,   194 
network  of  origin,  184,  186 
nodes  or  glands,  186 
parts  of,  183 

of  peri  dental  membrane,  195 
of  pulp,  197 
Schweitzer  on,  197 
substernomastoid  glands,  190 
thoracic  duct,  182 
of  tongue,  197 

anterior  apical,  197 
marginal,  198 
median  or  central,  199 
posterior  or  basal,  198 


M 


MACERATION,  476 

Magitot,  330 

Magnesium  phosphate  in  dentin,  136 

in  enamel,  38 
Mallory,  277 

Mammalian  segmentation,  310 
Mandible,  buds  of,  317 

distribution  of  bone  of,  341 

growth  of,  335 

structure  of,  336 
Marginal  ridges  as  areas  of  weakness, 

127 

Massa,  183 
Matrix  of  bone,  210 

of  dentin,  136 
Maturation,  303 
Maxilla,  palatal  process  of,  319 

structure  of,  336 
Maxillary  sinus,  347 
McMurrich,  304 
Meckel's  cartilage,  325 
Membrana  eboris,  169 
Meroblastic  segmentation,  308 
Methods  of  embedding,  431,  435 
Methyl  green,  443 
Meyer's  fixative,  439 
Miller  on  caries,  57 
Molar,  permanent  origin  of,  327 


452 


Morris'  staining  dish,  433 

Mounting  of  specimens,  429 

Mouth  cavity,  288,  314,  319 
epithelium  of,  288 
formation  of,  314,  319 
glands  of,  290 
mucous  membrane  of,  288 
nerve  endings  in,  290 
separation  from  nose  cavity, 

319 

submucosa  of,  289 
taste-buds  of,  295 
tongue,  291 

Mucous  glands,  290 

M tillers  fluid,  441 

Mummery,  177 


N 


NASMYTH'S  membrane,  74 
Nerve  fibers  in  dentin,  178 

in  peridental  membrane,  271 

in  pulp,  177 
Newman's  sheaths,  137 
Nissl,  279 
Northwestern    University    Dental 

School,  274,  280 
Notochord,  19 


OBLIQUE  group  of  fibers,  241 
Odontoblasts,  113,  166 
Oil  of  bergamot,  445 

of  cedarwood,  445 

of  origanum,  455 

Oocytes,  primary  and  secondary,  303 
Oogonia,  303 

Osteoblastsof  peridental  membrane,254 
Osteoclasts,  276 

as  absorbent  organ,  287 

in  burrowing  canals,  255 

in  cementum,  278 

in  dentin,  281 

function  of,  276 

origin  of  276 

in  peridental  membrane,  278 
Owen's  odontography,  75,  150 


PALATE,  formation  of,  319 

soft,  295 
Papillae  of  gingivse,  266 

of  gum,  289 

of  lip,  291 

of  tongue,  293 
Paraffin,  cutting  of  432 


Paraffin,  embedding  in,  431 
kinds  of,  431 

method,  summary  of,  434 
Pathological  absorption  of  permanent 

tooth  roots,  282 
Paul,  75 

Peridental  membrane,  237 
absorption  of,  278 
arrangement  of  fibers  of,  240 
blood  supply  of,  267 
cellular  elements  of,  250 
cementoblasts  in,  251 
changes  in,  with  age,  272 
classification  of  fibers  of,  240 

Black  on,  241 
comparison  with  capsules  of 

organ,  238 
with  periosteum,  238 
definition  of,  237 
division  of,  238 
epithelial  structure  in,  260 
Black  on,  260 
von  Brunn  on,  260 
fibroblasts  in,  250 
fibrous  tissue  of,  240 
function  of,  240 
gland  of  Serres,  264 
lymphatics  of,  195,  270 
nerves  of,  271 
nomenclature  of,  237 
Pacinian  corpuscles  in,  271 
practical  consideration  of,  274 
preparation  of  material  in,  274 
principal  fibers  of,  240 

classification  of,  241 
relation  of  cementoblasts  in, 

to  cure  of  pockets,  253 
structural  elements  of,  240 
Periosteum,  appearance  of,  223 
attached  complex,  229 

simple,  227 
Black  on,  222 
classification  of,  222 
definition  of,  222 
function  of,  222 
layers  of,  224 

relation  of  attachment  of,  to  bur- 
rowing pus,  224 
structural  elements  of,  224 
unattached  complex,  226 

simple,  225 
Permanent  molars,  first,  327 

second  and  third,  330 
teeth,  origin  of,  324 
Physiological  absorption  of  tooth  root, 

279 

Picric  acid,  440 
Placoid  scales,  22 
Pleurodont,  235 
Point  finder,  409 
Polar  bodies,  303 


INDEX 


453 


Prentiss,  276 

Preparation  of  material,  411 
Preserving,  430 
Processes  globularis,  319 
Pulp,  164 

arteries  of,  171 

cells,  connective  tissue,  170 
specialized,  166 

function  of,  164 

intercellular  substance  of,  171 

layer  of  Weil,  171 

lymphatics  of,  175 

membrana  eboris  of,  169 


QUAIN,  314 


RAPIDITY  of  grinding,  414 
Reattachment  of  tissue  to  tooth  roots, 

253 
Relation  of  enamel  to  formative  organ, 

40 

of  nucleus  to  cytoplasm,  299 
of  tooth  to  bone,  26 
Removal  of  cover-glass  from  grinding 

disk,  418 

Retzius,  bands  of,  70 
Rose,  179 


S 


SAFEANIN,  442 

Salter,  168,  283 

Schaffer,  276 

Schweitzer,  197 

Secondary  curves  of  dentinal  tubules, 

139 
dentin,  150 

and  cementum,  study  of,  391 
tubules  of,  150 
Sectioning  methods,  431 
Segmentation,  306 
holoblastic,  306 
mammalian,  310 
meroblastic,  308 
Serial  sections,  437 
Serres,  gland  of,  264 
Sertoli,  304 
Sharpey's  fibers,  160 
Sheaths  of  Newman,  137 
Sheep  teeth,  absorption  of,  278 
Silver  nitrate,  443,  444 
injection,  444 

Skillen's    stain    of   granular    layer    of 
Tomes,  145 


Slicing  mechanism,  422 
Southwell's  experiment,  146 
Spatter  guard,  411 
Spermatids,  304 
Spermatocyte,  304 
Spermatogenesis,  304 
Spermatogonia,  304 
Spermatozoa,  304 
Staining  agents,  442 

of  celloidin  sections,  436 
Stellate  reticulum,  323 
Stohr,  228 
Stowell,  171 

Stratum  intermedium,  332 
Subperiosteal  bone,  211 

and  cementum,  393 


TASTE-BUDS,  295 
Teasing,  42^ 

Teeth,  attachment  of,  230 
for  grinding,  377 
relation  of,  to  bone,  26 
Thecodont  attachment,  235 
Tissue    changes    with    movement    of 

teeth,  368 

Tomes,  Charles,  39,  230 
granular  laver  of,  145 
John,  145,  276 
Tongue,  290 

epithelium  of,  294 
glands  of,  290 
muscles  of,  291 
papillae  of,  293 

circumvallate,  293 
filiform,  293 
fungiform,  293 
taste-buds  of,  295 
tonsils  of,  297 
Tonsils,  296 
lingual,  297 
palatal,  297 
pharyngeal,  297 
Tooth  attachment,  230 
ankylosis,  233 
fibrous,  231 
gomphosis,  236 
hinge-joint,  231 
germs,  beginning  of  formation  of, 

325 

of  permanent  teeth,  324 
origin  of,  324 
time  of,  325 

of  temporary  teeth,  322 
Traneeptal  group  of  fibers,  241 
Transmission,  vehicle  of,  301 
Transverse  sections  of  tooth  roots,  380 


454 


INDEX 


VITAL   function    of   peridental    mer 

brane,  241 
of  pulp,  164 

Von  Bibra  enamel  analysis,  38 
Von  Brunn  and  epithelial  cords,  260 
Von  Giesen  stain,  443 


W 


WALDEYER,  75 
Walkoff,  337 


Washing  of  tissue,  430 

Waste  water,  411 

Water  of  crystallization  in  enamel,  39 

Watering  stones  in  grinding,  410 

Weed,  279 

Wegener,  276 

Weil,  171 

Williams  on  enamel,  39 

Wilson,  305 


ZENKER'S  solution,  441 


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